Clean up Create/DropReplicationSlot query buffer

CreateReplicationSlot() and DropReplicationSlot() were not cleaning up
the query buffer in some cases (mostly error conditions) which meant a
small leak.  Not generally an issue as the error case would result in an
immediate exit, but not difficult to fix either and reduces the number
of false positives from code analyzers.

In passing, also add appropriate PQclear() calls to RunIdentifySystem().

Pointed out by Coverity.
Fix array overrun in ecpg's version of ParseDateTime().

The code wrote a value into the caller's field[] array before checking
to see if there was room, which of course is backwards.  Per report from
Michael Paquier.

I fixed the equivalent bug in the backend's version of this code way back
in 630684d3a130bb93, but failed to think about ecpg's copy.  Fortunately
this doesn't look like it would be exploitable for anything worse than a
core dump: an external attacker would have no control over the single word
that gets written.
Fix typo in elog message.