bus-message: fix skipping of array fields in !gvariant messages

We copied part of the string into a buffer that was off by two.
If the element signature had length one, we'd copy 0 bytes and crash when
looking at the "first" byte. Otherwise, we would crash because strncpy would
not terminate the string.

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

@@ -4964,12 +4964,12 @@ static int message_skip_fields(
 
                         assert(l >= 1);
                         {
-                                char sig[l-1], *s;
+                                char sig[l + 1], *s = sig;
                                 uint32_t nas;
                                 int alignment;
 
-                                strncpy(sig, *signature + 1, l-1);
-                                s = sig;
+                                strncpy(sig, *signature + 1, l);
+                                sig[l] = '\0';
 
                                 alignment = bus_type_get_alignment(sig[0]);
                                 if (alignment < 0)