smoothstreamingenc: Add a missing "goto fail"

This goto wasn't necessary originally, but it should have been
added when the write_manifest call was added in 8e276378.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/smoothstreamingenc.c

@@ -394,6 +394,7 @@ static int ism_write_header(AVFormatContext *s)
     if (!c->has_video && c->min_frag_duration <= 0) {
         av_log(s, AV_LOG_WARNING, "no video stream and no min frag duration set\n");
         ret = AVERROR(EINVAL);
+        goto fail;
     }
     ret = write_manifest(s, 0);