avcodec: only set min/maxrate if rc-buffer-size is set

(cherry picked from commit 50a0ceae0ffa535512c5fd8e0952f277b1732458)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/avcodec/encoder.c

@@ -590,8 +590,11 @@ int OpenEncoder( vlc_object_t *p_this )
         else
         {
             p_context->rc_qsquish = 1.0;
+            if( p_sys->i_rc_buffer_size )
+            {
                 p_context->rc_max_rate = p_enc->fmt_out.i_bitrate;
                 p_context->rc_min_rate = p_enc->fmt_out.i_bitrate;
+            }
             p_context->rc_buffer_size = p_sys->i_rc_buffer_size;
             /* This is from ffmpeg's ffmpeg.c : */
             p_context->rc_initial_buffer_occupancy