mingw_utime(): handle NULL times parameter

POSIX sayeth:

  "If times is a null pointer, the access and modification
   times of the file shall be set to the current time."

Let's do so.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -304,8 +304,13 @@ int mingw_utime (const char *file_name, const struct utimbuf *times)
 		goto revert_attrs;
 	}
 
-	time_t_to_filetime(times->modtime, &mft);
-	time_t_to_filetime(times->actime, &aft);
+	if (times) {
+		time_t_to_filetime(times->modtime, &mft);
+		time_t_to_filetime(times->actime, &aft);
+	} else {
+		GetSystemTimeAsFileTime(&mft);
+		aft = mft;
+	}
 	if (!SetFileTime((HANDLE)_get_osfhandle(fh), NULL, &aft, &mft)) {
 		errno = EINVAL;
 		rc = -1;