avformat/vividas: check if size is enough big in xor_block

libavformat/vividas.c

@@ -129,11 +129,12 @@ static void xor_block(void *p1, void *p2, unsigned size, int key, unsigned *key_
 
     size >>= 2;
 
-    while (size--) {
+    while (size > 0) {
         *d2 = *d1 ^ k;
         k += key;
         d1++;
         d2++;
+        size--;
     }
 
     *key_ptr = k;