ffmpeg_opt: Fix forcing fourccs

Fixes Ticket4682
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 8750aef3)

ffmpeg_opt.c

@@ -1203,6 +1203,7 @@ static OutputStream *new_output_stream(OptionsContext *o, AVFormatContext *oc, e
         uint32_t tag = strtol(codec_tag, &next, 0);
         if (*next)
             tag = AV_RL32(codec_tag);
+        ost->st->codec->codec_tag =
         ost->enc_ctx->codec_tag = tag;
     }