wma: Check channel number before init.

Fixes Ticket240

Based on patch by ami_stuff
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/wmadec.c

@@ -109,6 +109,11 @@ static int wma_decode_init(AVCodecContext * avctx)
         }
     }
 
+    if(avctx->channels > MAX_CHANNELS){
+        av_log(avctx, AV_LOG_ERROR, "Invalid number of channels (%d)\n", avctx->channels);
+        return -1;
+    }
+
     if(ff_wma_init(avctx, flags2)<0)
         return -1;