Don't leak malloc'd error string in libpqrcv_check_conninfo().

Don't leak malloc'd error string in libpqrcv_check_conninfo().

We leaked the error report from PQconninfoParse, when there was
one.  It seems unlikely that real usage patterns would repeat
the failure often enough to create serious bloat, but let's
back-patch anyway to keep the code similar in all branches.

Found via valgrind testing.
Back-patch to v10 where this code was added.

Discussion: https://postgr.es/m/3816764.1616104288@sss.pgh.pa.us