wmavoice: fix stack overread.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/wmavoice.c

@@ -1440,8 +1440,7 @@ static int synth_frame(AVCodecContext *ctx, GetBitContext *gb, int frame_idx,
     int pitch[MAX_BLOCKS], last_block_pitch;
 
     /* Parse frame type ("frame header"), see frame_descs */
-    int bd_idx = s->vbm_tree[get_vlc2(gb, frame_type_vlc.table, 6, 3)],
-        block_nsamples = MAX_FRAMESIZE / frame_descs[bd_idx].n_blocks;
+    int bd_idx = s->vbm_tree[get_vlc2(gb, frame_type_vlc.table, 6, 3)], block_nsamples;
 
     if (bd_idx < 0) {
         av_log(ctx, AV_LOG_ERROR,
@@ -1449,6 +1448,8 @@ static int synth_frame(AVCodecContext *ctx, GetBitContext *gb, int frame_idx,
         return -1;
     }
 
+    block_nsamples = MAX_FRAMESIZE / frame_descs[bd_idx].n_blocks;
+
     /* Pitch calculation for ACB_TYPE_ASYMMETRIC ("pitch-per-frame") */
     if (frame_descs[bd_idx].acb_type == ACB_TYPE_ASYMMETRIC) {
         /* Pitch is provided per frame, which is interpreted as the pitch of