pickaxe: plug diff filespec leak with empty needle

Check first for the unlikely case of an empty needle string and only
then populate the filespec, lest we leak it.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-pickaxe.c

@@ -152,10 +152,10 @@ static unsigned int contains(struct diff_filespec *one,
 	unsigned int cnt;
 	unsigned long sz;
 	const char *data;
-	if (diff_populate_filespec(one, 0))
-		return 0;
 	if (!len)
 		return 0;
+	if (diff_populate_filespec(one, 0))
+		return 0;
 
 	sz = one->size;
 	data = one->data;