unistr.c: Fix use-after-free in 'ntfs_uppercase_mbs'.

If 'utf8_to_unicode' throws an error due to an invalid UTF-8 sequence,
then 'n' will be less than 0 and the loop will terminate without storing
anything in '*t'. After the loop the uppercase string's allocation is
freed, however after it is freed it is unconditionally accessed through
'*t', which points into the freed allocation, for the purpose of NULL-
terminating the string. This leads to a use-after-free.
Fixed by only NULL-terminating the string when no error has been thrown.

Thanks for Jeffrey Bencteux for reporting this issue:
#84

libntfs-3g/unistr.c

@@ -1189,8 +1189,9 @@ char *ntfs_uppercase_mbs(const char *low,
 			free(upp);
 			upp = (char*)NULL;
 			errno = EILSEQ;
+		} else {
+			*t = 0;
 		}
-		*t = 0;
 	}
 	return (upp);
 }