mingw: do not set errno to 0 on success

Currently do_lstat always sets errno to 0 on success. This incorrectly
overwrites previous errors.

Fetch the error-code into a temporary variable instead, and assign that
to errno on failure.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -195,9 +195,10 @@ static inline time_t filetime_to_time_t(const FILETIME *ft)
  */
 static int do_lstat(const char *file_name, struct stat *buf)
 {
+	int err;
 	WIN32_FILE_ATTRIBUTE_DATA fdata;
 
-	if (!(errno = get_file_attr(file_name, &fdata))) {
+	if (!(err = get_file_attr(file_name, &fdata))) {
 		buf->st_ino = 0;
 		buf->st_gid = 0;
 		buf->st_uid = 0;
@@ -211,6 +212,7 @@ static int do_lstat(const char *file_name, struct stat *buf)
 		buf->st_ctime = filetime_to_time_t(&(fdata.ftCreationTime));
 		return 0;
 	}
+	errno = err;
 	return -1;
 }