check for qscale==0 (fixes 1/0 on one corrupted stream)

Originally committed as revision 1574 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/msmpeg4.c

@@ -1228,6 +1228,10 @@ return -1;
 }
 #endif
     s->qscale = get_bits(&s->gb, 5);
+    if(s->qscale==0){
+        fprintf(stderr, "invalid qscale\n");
+        return -1;
+    }
 
     if (s->pict_type == I_TYPE) {
         code = get_bits(&s->gb, 5);