Use ereport(ERROR) instead of Assert() to emit syncrep_parser error.

The existing code would either Assert or generate an invalid
SyncRepConfig variable, neither of which is desirable. A regular
error should be thrown instead.

This commit silences compiler warning in non assertion-enabled builds.

Per report from Jeff Janes.
Suggested fix by Tom Lane.

src/backend/replication/syncrep.c

@@ -886,9 +886,14 @@ SyncRepUpdateConfig(void)
 	 */
 	syncrep_scanner_init(SyncRepStandbyNames);
 	parse_rc = syncrep_yyparse();
-	Assert(parse_rc == 0);
 	syncrep_scanner_finish();
 
+	if (parse_rc != 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_SYNTAX_ERROR),
+				 errmsg_internal("synchronous_standby_names parser returned %d",
+								 parse_rc)));
+
 	SyncRepConfig = syncrep_parse_result;
 	syncrep_parse_result = NULL;
 }