mp3dec: Fix a heap-buffer-overflow

In some cases, what is left to read from ptr is smaller than EXTRABYTES.

Based on a patch by Thierry Foucu <tfoucu@gmail.com>.
Signed-off-by: Alex Converse <alex.converse@gmail.com>

libavcodec/mpegaudiodec.c

@@ -1378,16 +1378,17 @@ static int mp_decode_layer3(MPADecodeContext *s)
     if (!s->adu_mode) {
         int skip;
         const uint8_t *ptr = s->gb.buffer + (get_bits_count(&s->gb)>>3);
+        int extrasize = av_clip(get_bits_left(&s->gb) >> 3, 0, EXTRABYTES);
         assert((get_bits_count(&s->gb) & 7) == 0);
         /* now we get bits from the main_data_begin offset */
         av_dlog(s->avctx, "seekback: %d\n", main_data_begin);
     //av_log(NULL, AV_LOG_ERROR, "backstep:%d, lastbuf:%d\n", main_data_begin, s->last_buf_size);
 
-        memcpy(s->last_buf + s->last_buf_size, ptr, EXTRABYTES);
+        memcpy(s->last_buf + s->last_buf_size, ptr, extrasize);
         s->in_gb = s->gb;
         init_get_bits(&s->gb, s->last_buf, s->last_buf_size*8);
 #if !UNCHECKED_BITSTREAM_READER
-        s->gb.size_in_bits_plus8 += EXTRABYTES * 8;
+        s->gb.size_in_bits_plus8 += extrasize * 8;
 #endif
         s->last_buf_size <<= 3;
         for (gr = 0; gr < nb_granules && (s->last_buf_size >> 3) < main_data_begin; gr++) {