mpegaudiodec: dont memcpy() more than needed.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegaudiodec.c

@@ -1380,8 +1380,7 @@ static int mp_decode_layer3(MPADecodeContext *s)
 
     if (!s->adu_mode) {
         const uint8_t *ptr = s->gb.buffer + (get_bits_count(&s->gb)>>3);
-        int extrasize = av_clip(get_bits_left(&s->gb) >> 3, 0,
-                                FFMAX(0, LAST_BUF_SIZE - s->last_buf_size));
+        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);
@@ -1391,7 +1390,7 @@ static int mp_decode_layer3(MPADecodeContext *s)
         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 += extrasize * 8;
+        s->gb.size_in_bits_plus8 += FFMAX(extrasize, LAST_BUF_SIZE - s->last_buf_size) * 8;
 #endif
         skip_bits_long(&s->gb, 8*(s->last_buf_size - main_data_begin));
     }