msdosfs: long names of files are created incorrectly.

This fixes a regression that happened in r120492 (2003) where libkiconv
was introduced and we went from checking unlen to checking for '\0'.

PR:		111843
Patch by:	Damjan Jovanovic
MFC after:	1 week

sys/fs/msdosfs/msdosfs_conv.c

@@ -570,7 +570,7 @@ unix2winfn(const u_char *un, size_t unlen, struct winentry *wep, int cnt,
 		if (!code)
 			end = WIN_LAST;
 	}
-	if (*un == '\0')
+	if (!unlen)
 		end = WIN_LAST;
 	wep->weCnt |= end;
 	return !end;