mpc8: fix maximum bands handling

In Musepack SV8 codec property tell the maximum nonzero band, but every
frame codes maximum band as a limit (i.e. strictly less than given value).
Synthesis also expects maximum nonzero band, so there's a need to convert
frame maximum band limit value.

libavcodec/mpc8.c

@@ -270,7 +270,7 @@ static int mpc8_decode_frame(AVCodecContext * avctx, void *data,
         maxband = c->last_max_band + get_vlc2(gb, band_vlc.table, MPC8_BANDS_BITS, 2);
         if(maxband > 32) maxband -= 33;
     }
-    if(maxband > c->maxbands)
+    if(maxband > c->maxbands + 1)
         return AVERROR_INVALIDDATA;
     c->last_max_band = maxband;
 
@@ -405,7 +405,8 @@ static int mpc8_decode_frame(AVCodecContext * avctx, void *data,
         }
     }
 
-    ff_mpc_dequantize_and_synth(c, maxband, c->frame.data[0], avctx->channels);
+    ff_mpc_dequantize_and_synth(c, maxband - 1, c->frame.data[0],
+                                avctx->channels);
 
     c->cur_frame++;