tiffenc: fix integer overflow

Fixes CID700699
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/tiffenc.c

@@ -137,11 +137,11 @@ static void add_entry(TiffEncoderContext * s,
     bytestream_put_le16(&entries_ptr, type);
     bytestream_put_le32(&entries_ptr, count);
 
-    if (type_sizes[type] * count <= 4) {
+    if (type_sizes[type] * (int64_t)count <= 4) {
         tnput(&entries_ptr, count, ptr_val, type, 0);
     } else {
         bytestream_put_le32(&entries_ptr, *s->buf - s->buf_start);
-        check_size(s, count * type_sizes2[type]);
+        check_size(s, count * (int64_t)type_sizes2[type]);
         tnput(s->buf, count, ptr_val, type, 0);
     }