fftools/ffmpeg: Fix honor -r output option with streamcopy

Fix "ost->st->avg_frame_rate = ost->frame_rate" in streamcopy_init()
being reset to input's frame rate a few lines below.
Note that in current code, there are some discrepancies amongst the
muxers. For example, avienc relies on time_base, so it is not affected
by this patch, whereas mxfenc and matroskaenc do use avg_frame_rate,
so this patch fixes -r being honored.

In the updated fate test, the input is (wrongly) probed as 50fps. With
this patch, the correct value (25fps) is successfully forced with -r.

Signed-off-by: Nicolas Gaullier <nicolas.gaullier@cji.paris>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

fftools/ffmpeg_mux_init.c

@@ -1012,7 +1012,6 @@ static int streamcopy_init(const Muxer *mux, OutputStream *ost, AVDictionary **e
         else
             sar = par->sample_aspect_ratio;
         ost->st->sample_aspect_ratio = par->sample_aspect_ratio = sar;
-        ost->st->avg_frame_rate = ist->st->avg_frame_rate;
         ost->st->r_frame_rate = ist->st->r_frame_rate;
         break;
         }

tests/ref/fate/time_base

@@ -1 +1 @@
-b28d4ca13029fdc80a114b56467be9d7
+69ffc45e19ab070bc3e964d7b718fe53