avformat/mp3enc: Assert that the header we assembled is valid

Silences: CID1351343

The header is calculated by the code above the changed hunk, it is
thus asserted that the header is always correct.
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mp3enc.c

@@ -192,7 +192,8 @@ static int mp3_write_xing(AVFormatContext *s)
             return -1;
         header |= mask;
 
-        avpriv_mpegaudio_decode_header(&mpah, header);
+        ret = avpriv_mpegaudio_decode_header(&mpah, header);
+        av_assert0(ret >= 0);
         mp3->xing_offset = xing_offtbl[mpah.lsf == 1][mpah.nb_channels == 1] + 4;
         bytes_needed     = mp3->xing_offset + XING_SIZE;