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.