mpeg4: adjust dummy frame threashold for packed divx.

Fixes Ticket427

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 3e7e1f1509)

libavcodec/h263dec.c

@@ -681,7 +681,7 @@ frame_end:
         int current_pos= s->gb.buffer == s->bitstream_buffer ? 0 : (get_bits_count(&s->gb)>>3);
         int startcode_found=0;
 
-        if(buf_size - current_pos > 5){
+        if(buf_size - current_pos > 7){
             int i;
             for(i=current_pos; i<buf_size-4; i++){
                 if(buf[i]==0 && buf[i+1]==0 && buf[i+2]==1 && buf[i+3]==0xB6){