avcodec/mpc8: Correct end truncation

Fixes Ticket5478
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/mpc8.c

@@ -415,10 +415,14 @@ static int mpc8_decode_frame(AVCodecContext * avctx, void *data,
     c->cur_frame++;
 
     c->last_bits_used = get_bits_count(gb);
-    if(get_bits_left(gb) < 8) // we have only padding left
-        c->last_bits_used = buf_size << 3;
     if(c->cur_frame >= c->frames)
         c->cur_frame = 0;
+    if(c->cur_frame == 0 && get_bits_left(gb) < 8) {// we have only padding left
+        c->last_bits_used = buf_size << 3;
+    } else if (get_bits_left(gb) < 0) {
+        av_log(avctx, AV_LOG_ERROR, "Overread %d\n", -get_bits_left(gb));
+        c->last_bits_used = buf_size << 3;
+    }
 
     *got_frame_ptr = 1;