Fix inconsistencies in SRF checks of pg_config() and string_to_table()

The execution paths of those functions have been using a set of checks
inconsistent with any other SRF function:
- string_to_table() missed a check on expectedDesc, the tuple descriptor
expected by the caller, that should never be NULL.  Introduced in
66f1630.
- pg_config() should check for a ReturnSetInfo, and expectedDesc cannot
be NULL.  Its error messages were also inconsistent.  Introduced in
a5c43b8.

Extracted from a larger patch by the same author, in preparation for a
larger patch set aimed at refactoring the way tuplestores are created
and checked in SRF functions.

Author: Melanie Plageman
Reviewed-by: Justin Pryzby
Discussion: https://postgr.es/m/CAAKRu_azyd1Z3W_r7Ou4sorTjRCs+PxeHw1CWJeXKofkE6TuZg@mail.gmail.com

src/backend/utils/adt/varlena.c

@@ -4839,7 +4839,8 @@ text_to_table(PG_FUNCTION_ARGS)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("set-valued function called in context that cannot accept a set")));
-	if (!(rsi->allowedModes & SFRM_Materialize))
+	if (!(rsi->allowedModes & SFRM_Materialize) ||
+		rsi->expectedDesc == NULL)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("materialize mode required, but it is not allowed in this context")));

src/backend/utils/misc/pg_config.c

@@ -37,11 +37,15 @@ pg_config(PG_FUNCTION_ARGS)
 	int			i = 0;
 
 	/* check to see if caller supports us returning a tuplestore */
-	if (!rsinfo || !(rsinfo->allowedModes & SFRM_Materialize))
+	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
 		ereport(ERROR,
-				(errcode(ERRCODE_SYNTAX_ERROR),
-				 errmsg("materialize mode required, but it is not "
-						"allowed in this context")));
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("set-valued function called in context that cannot accept a set")));
+	if (!(rsinfo->allowedModes & SFRM_Materialize) ||
+		rsinfo->expectedDesc == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("materialize mode required, but it is not allowed in this context")));
 
 	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
 	oldcontext = MemoryContextSwitchTo(per_query_ctx);