Fix issue 2093: pfree() called with a NULL pointer

[jrgemignani]

Fixed issue 2093 where pfree() was called with a NULL pointer.

The issue is due to some confusion with pfree(). There are 2 definitions for it, one that checks for a passed NULL and the other which does not.

Created a function, pfree_if_not_null(), to check for NULL and call pfree() if a NULL wasn't passed.

Modified the pfree references in the following files -

src/backend/commands/label_commands.c
src/backend/executor/cypher_merge.c
src/backend/executor/cypher_set.c
src/backend/parser/ag_scanner.l
src/backend/parser/cypher_analyze.c
src/backend/parser/cypher_expr.c
src/backend/parser/cypher_gram.y
src/backend/parser/cypher_parse_agg.c
src/backend/utils/adt/age_global_graph.c
src/backend/utils/adt/age_graphid_ds.c
src/backend/utils/adt/age_session_info.c
src/backend/utils/adt/age_vle.c
src/backend/utils/adt/agtype.c
src/backend/utils/adt/agtype_gin.c
src/backend/utils/adt/agtype_raw.c
src/backend/utils/adt/agtype_util.c
src/backend/utils/load/ag_load_edges.c
src/backend/utils/load/ag_load_labels.c
src/include/utils/age_graphid_ds.h
src/include/utils/age_session_info.h
src/include/utils/agtype.h

Added regression tests for the original issue.

[MironAtHome]

Just in case, at least in later edition of postgres, internal call to pfree->pg_free resolves to following:
F:\HL\dev\git\postgresql\postgres\src\common\fe_memutils.c

void
pg_free(void *ptr)
{
	if (ptr != NULL)
		free(ptr);
}

My check for branch REL_11_STABLE seems to confirm. Double - checking.
Yup, looks same:
https://github.com/postgres/postgres/blob/REL_11_STABLE/src/common/fe_memutils.c