roqvideodec: fix a potential infinite loop in roqvideo_decode_frame().

When there is just 1 byte remanining in the buffer, nothing will be read
and the loop will continue forever. Check that there are at least 8
bytes, which are always read at the beginning.

CC:libav-stable@libav.org

libavcodec/roqvideodec.c

@@ -44,7 +44,7 @@ static void roqvideo_decode_frame(RoqContext *ri)
     roq_qcell *qcell;
     int64_t chunk_start;
 
-    while (bytestream2_get_bytes_left(&ri->gb) > 0) {
+    while (bytestream2_get_bytes_left(&ri->gb) >= 8) {
         chunk_id   = bytestream2_get_le16(&ri->gb);
         chunk_size = bytestream2_get_le32(&ri->gb);
         chunk_arg  = bytestream2_get_le16(&ri->gb);