mp3dec: perform I/S and M/S only when frame mode is joint stereo.

Looks like some LAME versions produce dual stereo mode MP3s with
flags for intensity and middle stereo set. In this mode those flags
should be ignored like the reference decoder and derived ones do.

libavcodec/mpegaudiodec.c

@@ -1533,7 +1533,7 @@ static int mp_decode_layer3(MPADecodeContext *s)
             huffman_decode(s, g, exponents, bits_pos + g->part2_3_length);
         } /* ch */
 
-        if (s->nb_channels == 2)
+        if (s->mode == MPA_JSTEREO)
             compute_stereo(s, &s->granules[0][gr], &s->granules[1][gr]);
 
         for (ch = 0; ch < s->nb_channels; ch++) {