fixing q>0.0 assert failure caused by overflow of variance for b frames

Originally committed as revision 1120 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/motion_est.c

@@ -1558,7 +1558,7 @@ void ff_estimate_b_frame_motion(MpegEncContext * s,
             score=fbmin;
             type= MB_TYPE_BIDIR;
         }
-        score= (score*score + 128*256)>>16;
+        score= ((unsigned)(score*score + 128*256))>>16;
         s->mc_mb_var_sum += score;
         s->mc_mb_var[mb_y*s->mb_width + mb_x] = score; //FIXME use SSD
     }