Avoid throwing away the error message in syncrep_yyerror.

Commit 473a575e05 purported to make this
function stash the error message in *syncrep_parse_result_p, but
it didn't actually.

As a result, an attempt to set synchronous_standby_names to any value
that does not parse resulted in a generic "parser failed." message
rather than anything more specific. This fixes that.

Discussion: http://postgr.es/m/CA+TgmoYF9wPNZ-Q_EMfib_espgHycY-eX__6Tzo2GpYpVXqCdQ@mail.gmail.com
Backpatch-through: 18

src/backend/replication/syncrep_scanner.l

@@ -157,17 +157,16 @@ syncrep_yyerror(SyncRepConfigData **syncrep_parse_result_p, char **syncrep_parse
 {
 	struct yyguts_t *yyg = (struct yyguts_t *) yyscanner;	/* needed for yytext
 															 * macro */
-	char *syncrep_parse_error_msg = *syncrep_parse_error_msg_p;
 
 	/* report only the first error in a parse operation */
-	if (syncrep_parse_error_msg)
+	if (*syncrep_parse_error_msg_p)
 		return;
 	if (yytext[0])
-		syncrep_parse_error_msg = psprintf("%s at or near \"%s\"",
-										   message, yytext);
+		*syncrep_parse_error_msg_p = psprintf("%s at or near \"%s\"",
+											  message, yytext);
 	else
-		syncrep_parse_error_msg = psprintf("%s at end of input",
-										   message);
+		*syncrep_parse_error_msg_p = psprintf("%s at end of input",
+											  message);
 }
 
 void