mpeg4videodec: only allow a positive length

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit b3408ae4)

Conflicts:

	libavcodec/mpeg4videodec.c

libavcodec/mpeg4videodec.c

@@ -186,13 +186,13 @@ static int mpeg4_decode_sprite_trajectory(MpegEncContext *s, GetBitContext *gb)
         int x=0, y=0;
 
         length= get_vlc2(gb, sprite_trajectory.table, SPRITE_TRAJ_VLC_BITS, 3);
-        if(length){
+        if(length > 0){
             x= get_xbits(gb, length);
         }
         if(!(s->divx_version==500 && s->divx_build==413)) skip_bits1(gb); /* marker bit */
 
         length= get_vlc2(gb, sprite_trajectory.table, SPRITE_TRAJ_VLC_BITS, 3);
-        if(length){
+        if(length > 0){
             y=get_xbits(gb, length);
         }
         skip_bits1(gb); /* marker bit */