Fix broken vbv_buffer_init handling in libx264.c

Due to being pants-on-head retarded, libavcodec defaults this to zero, which
results in broken output.  This didn't affect ffmpeg.c, which sets it itself,
but caused problems for other calling apps using VBV.
(cherry picked from commit f7f8120f)

libavcodec/libx264.c

@@ -277,12 +277,11 @@ static av_cold int X264_init(AVCodecContext *avctx)
     if (avctx->level > 0)
         x4->params.i_level_idc = avctx->level;
 
-    if ((avctx->rc_buffer_size != 0) &&
+    if (avctx->rc_buffer_size && avctx->rc_initial_buffer_occupancy &&
         (avctx->rc_initial_buffer_occupancy <= avctx->rc_buffer_size)) {
         x4->params.rc.f_vbv_buffer_init =
             (float)avctx->rc_initial_buffer_occupancy / avctx->rc_buffer_size;
-    } else
-        x4->params.rc.f_vbv_buffer_init = 0.9;
+    }
 
     x4->params.rc.b_mb_tree               = !!(avctx->flags2 & CODEC_FLAG2_MBTREE);
     x4->params.rc.f_ip_factor             = 1 / fabs(avctx->i_quant_factor);