Fix some inconsistencies with memory freeing in pg_createsubscriber

Fix some inconsistencies with memory freeing in pg_createsubscriber

The correct function documented to free the memory allocated for the
result returned by PQescapeIdentifier() and PQescapeLiteral() is
PQfreemem().  pg_createsubscriber.c relied on pg_free() instead, which
is not incorrect as both do a free() internally, but inconsistent with
the documentation.

While on it, this commit fixes a small memory leak introduced by
4867f8a555ce, as the code of pg_createsubscriber makes this effort.

Author: Ranier Vilela
Reviewed-by: Euler Taveira
Discussion: https://postgr.es/m/CAEudQAp=AW5dJXrGLbC_aZg_9nOo=42W7uLDRONFQE-gcgnkgQ@mail.gmail.com
Backpatch-through: 17