Check rc_buffer_size value using integer arithmetic

Using floating-point here can cause erroneous rejection of
parameters due to rounding errors leading to a slightly too
large result.

This fixes the mxf regression test with gcc 4.5 on x86_32.

Originally committed as revision 25060 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo_enc.c

@@ -351,7 +351,7 @@ av_cold int MPV_encode_init(AVCodecContext *avctx)
         av_log(avctx, AV_LOG_INFO, "impossible bitrate constraints, this will fail\n");
     }
 
-    if(avctx->rc_buffer_size && avctx->bit_rate*av_q2d(avctx->time_base) > avctx->rc_buffer_size){
+    if(avctx->rc_buffer_size && avctx->bit_rate*(int64_t)avctx->time_base.num > avctx->rc_buffer_size * (int64_t)avctx->time_base.den){
         av_log(avctx, AV_LOG_ERROR, "VBV buffer too small for bitrate\n");
         return -1;
     }