fileops: correct error return on p_lstat failures when mkdir

IIRC I got a strange return once from lstat, which translated in a weird
error class/message being reported. As a safety measure, enforce a -1 return in
that case.

src/fileops.c

@@ -476,6 +476,7 @@ int git_futils_mkdir(
 			break;
 		} else if (errno != ENOENT) {
 			git_error_set(GIT_ERROR_OS, "failed to stat '%s'", parent_path.ptr);
+			error = -1;
 			goto done;
 		}