core/load-fragment: reject overly long paths early

core/load-fragment: reject overly long paths early

No need to go through the specifier_printf() if the path is already too long in
the unexpanded form (since specifiers increase the length of the string in all
practical cases).

In the oss-fuzz test case, valgrind reports:
  total heap usage: 179,044 allocs, 179,044 frees, 72,687,755,703 bytes allocated
and the original config file is ~500kb. This isn't really a security issue,
since the config file has to be trusted any way, but just a matter of
preventing accidental resource exhaustion.

https://oss-fuzz.com/v2/issue/4651449704251392/6977

While at it, fix order of arguments in the neighbouring log_syntax() call.