Don't leak memory during failure exit from SelectConfigFiles().

Don't leak memory during failure exit from SelectConfigFiles().

Make sure the memory allocated by make_absolute_path() is freed
when SelectConfigFiles() fails.  Since all the callers will exit
immediately in that case, there's no practical gain here, but
silencing Valgrind leak complaints seems useful.  In any case,
it was inconsistent that only one of the failure exits did this.

Author: Aleksander Alekseev <aleksander@tigerdata.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAJ7c6TMByXE8dc7zDvDWTQjk6o-XXAdRg_RAg5CBaUOgFPV3LQ%40mail.gmail.com