Better error reporting if the link target is too long

This situation won't set errno, so using %m will give an incorrect
error message.

src/backend/utils/adt/misc.c

@@ -287,9 +287,12 @@ pg_tablespace_location(PG_FUNCTION_ARGS)
 	 */
 	snprintf(sourcepath, sizeof(sourcepath), "pg_tblspc/%u", tablespaceOid);
 	rllen =readlink(sourcepath, targetpath, sizeof(targetpath));
-	if (rllen < 0 || rllen >= sizeof(targetpath))
+	if (rllen < 0)
 		ereport(ERROR,
 				(errmsg("could not read symbolic link \"%s\": %m", sourcepath)));
+	else if (rllen >= sizeof(targetpath))
+		ereport(ERROR,
+				(errmsg("symbolic link \"%s\" target is too long", sourcepath)));
 	targetpath[rllen] = '\0';
 
 	PG_RETURN_TEXT_P(cstring_to_text(targetpath));