bus-socket: Fix line_begins() to accept word matching full string

The switch to memory_startswith() changed the logic to only look for a space or
NUL byte after the matched word, but matching the full size should also be
acceptable.

This changed the behavior of parsing of "AUTH\r\n", where m will be set to 4,
since even though the word will match, the check for it being followed by ' '
or NUL will make line_begins() return false.

Tested:

- Using netcat to connect to the private socket directly:
  $ echo -ne '\0AUTH\r\n' | sudo nc -U /run/systemd/private
  REJECTED EXTERNAL ANONYMOUS

- Running the Ignition blackbox test:
  $ sudo sh -c 'PATH=$PWD/bin/amd64:$PATH ./tests.test'
  PASS

Fixes: d27b725abf64a19a6b2f99332b663f17ad046771

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

@@ -246,10 +246,7 @@ static bool line_begins(const char *s, size_t m, const char *word) {
         const char *p;
 
         p = memory_startswith(s, m, word);
-        if (!p)
-                return false;
-
-        return IN_SET(*p, 0, ' ');
+        return p && (p == (s + m) || *p == ' ');
 }
 
 static int verify_anonymous_token(sd_bus *b, const char *p, size_t l) {