Release PQconninfoOptions array in GetDbnameFromConnectionOptions().

Release PQconninfoOptions array in GetDbnameFromConnectionOptions().

It wasn't getting freed in one code path, which Coverity identified as
a resource leak.  It's probably of little consequence, but re-ordering
the code into the correct sequence is no more work than dismissing the
complaint.  Minor oversight in commit a145f424d.

While here, improve the unreasonably clunky coding of
FindDbnameInConnParams: use of an output parameter is unnecessary
and prone to uninitialized-variable problems.