Fix incorrect unsigned->signed conversion.

Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -2040,7 +2040,8 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, uint8_t *data,
                 }
             } else {
                 MatroskaTrackEncoding *encodings = track->encodings.elem;
-                int offset = 0, pkt_size = lace_size[n];
+                int offset = 0;
+                uint32_t pkt_size = lace_size[n];
                 uint8_t *pkt_data = data;
 
                 if (pkt_size > size) {