fuzz: skip bus error map in bus_error_name_to_errno()

Fuzzing with AddressSanitizer reports an error here:
==11==ERROR: AddressSanitizer: global-buffer-overflow on address 0x7fe53f5497d8 at pc 0x7fe53ef055c9 bp 0x7ffd344e9380 sp 0x7ffd344e9378
READ of size 4 at 0x7fe53f5497d8 thread T0
SCARINESS: 27 (4-byte-read-global-buffer-overflow-far-from-bounds)
    #0 0x7fe53ef055c8 in bus_error_name_to_errno /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-error.c:118:24
    #1 0x7fe53ef0577b in bus_error_setfv /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-error.c:274:17
    #2 0x7fe53ef0595a in sd_bus_error_setf /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-error.c:284:21
    #3 0x561059 in manager_load_unit_prepare /work/build/../../src/systemd/src/core/manager.c
    #4 0x560680 in manager_load_unit /work/build/../../src/systemd/src/core/manager.c:1773:13
    #5 0x5d49a6 in unit_add_dependency_by_name /work/build/../../src/systemd/src/core/unit.c:2882:13
    #6 0x538996 in config_parse_unit_deps /work/build/../../src/systemd/src/core/load-fragment.c:152:21
    #7 0x6db771 in next_assignment /work/build/../../src/systemd/src/shared/conf-parser.c:155:32
    #8 0x6d697e in parse_line /work/build/../../src/systemd/src/shared/conf-parser.c:273:16
    #9 0x6d5c48 in config_parse /work/build/../../src/systemd/src/shared/conf-parser.c:390:21
    #10 0x535678 in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/fuzz/fuzz-unit-file.c:41:16
    #11 0x73bd60 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:517:13
    #12 0x73a39f in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) /src/libfuzzer/FuzzerLoop.cpp:442:3
    #13 0x73d9bc in fuzzer::Fuzzer::MutateAndTestOne() /src/libfuzzer/FuzzerLoop.cpp:650:19
    #14 0x73fa05 in fuzzer::Fuzzer::Loop(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, fuzzer::fuzzer_allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&) /src/libfuzzer/FuzzerLoop.cpp:773:5
    #15 0x71f75d in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:754:6
    #16 0x71285c in main /src/libfuzzer/FuzzerMain.cpp:20:10
    #17 0x7fe53da0482f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #18 0x430e68 in _start (/out/fuzz-unit-file+0x430e68)

0x7fe53f5497d8 is located 8 bytes to the right of global variable 'bus_common_errors' defined in '../../src/systemd/src/libsystemd/sd-bus/bus-common-errors.c:28:51' (0x7fe53f549300) of size 1232
SUMMARY: AddressSanitizer: global-buffer-overflow /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-error.c:118:24 in bus_error_name_to_errno
Shadow bytes around the buggy address:
  0x0ffd27ea12a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffd27ea12b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffd27ea12c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffd27ea12d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffd27ea12e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0ffd27ea12f0: 00 00 00 00 00 00 00 00 00 00 f9[f9]f9 f9 f9 f9
  0x0ffd27ea1300: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
  0x0ffd27ea1310: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
  0x0ffd27ea1320: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffd27ea1330: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffd27ea1340: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==11==ABORTING

but I think it's a false positive because of our low-level magic in how this
area is constructed.

src/libsystemd/sd-bus/bus-error.c

@@ -108,6 +108,7 @@ static int bus_error_name_to_errno(const char *name) {
                         }
 
         m = __start_BUS_ERROR_MAP;
+#ifndef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
         while (m < __stop_BUS_ERROR_MAP) {
                 /* For magic ELF error maps, the end marker might
                  * appear in the middle of things, since multiple maps
@@ -125,6 +126,7 @@ static int bus_error_name_to_errno(const char *name) {
 
                 m++;
         }
+#endif
 
         return EIO;
 }