Add an Assert that enum_cmp_internal() gets passed an FmgrInfo pointer.

If someone were to try to call one of the enum comparison functions
using DirectFunctionCallN, it would very likely seem to work, because
only in unusual cases does enum_cmp_internal() need to access the
typcache.  But once such a case occurred, code like that would crash
with a null pointer dereference.  To make an oversight of that sort
less likely to escape detection, add a non-bypassable Assert that
fcinfo->flinfo isn't NULL.

Discussion: https://postgr.es/m/25226.1487900067@sss.pgh.pa.us

src/backend/utils/adt/enum.c

@@ -263,6 +263,15 @@ enum_cmp_internal(Oid arg1, Oid arg2, FunctionCallInfo fcinfo)
 {
 	TypeCacheEntry *tcache;
 
+	/*
+	 * We don't need the typcache except in the hopefully-uncommon case that
+	 * one or both Oids are odd.  This means that cursory testing of code that
+	 * fails to pass flinfo to an enum comparison function might not disclose
+	 * the oversight.  To make such errors more obvious, Assert that we have a
+	 * place to cache even when we take a fast-path exit.
+	 */
+	Assert(fcinfo->flinfo != NULL);
+
 	/* Equal OIDs are equal no matter what */
 	if (arg1 == arg2)
 		return 0;
@@ -381,12 +390,7 @@ enum_cmp(PG_FUNCTION_ARGS)
 	Oid			a = PG_GETARG_OID(0);
 	Oid			b = PG_GETARG_OID(1);
 
-	if (a == b)
-		PG_RETURN_INT32(0);
-	else if (enum_cmp_internal(a, b, fcinfo) > 0)
-		PG_RETURN_INT32(1);
-	else
-		PG_RETURN_INT32(-1);
+	PG_RETURN_INT32(enum_cmp_internal(a, b, fcinfo));
 }
 
 /* Enum programming support functions */