Silence minor compiler warnings.

Ensure that ClassifyUtilityCommandAsReadOnly() has defined behavior
even if TransactionStmt.kind has a value that's not one of the
declared values for its enum.

Suppress warnings from compilers that don't know that elog(ERROR)
doesn't return, in ClassifyUtilityCommandAsReadOnly() and
jsonb_set_lax().

Per Coverity and buildfarm.

src/backend/utils/adt/jsonfuncs.c

@@ -4456,6 +4456,7 @@ jsonb_set_lax(PG_FUNCTION_ARGS)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("need delete_key, return_target, use_json_null, or raise_exception")));
+		return (Datum) 0;		/* silence stupider compilers */
 	}
 }