DirectorySave: save the mtime only if it is known

src/db/DirectorySave.cxx

@@ -43,6 +43,7 @@ void
 directory_save(FILE *fp, const Directory &directory)
 {
 	if (!directory.IsRoot()) {
+		if (directory.mtime != 0)
 			fprintf(fp, DIRECTORY_MTIME "%lu\n",
 				(unsigned long)directory.mtime);