wmadec: avoid infinit loop.

Fixes ticket183
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/wmadec.c

@@ -827,7 +827,7 @@ static int wma_decode_superframe(AVCodecContext *avctx,
         return 0;
     }
     if (buf_size < s->block_align)
-        return 0;
+        return AVERROR(EINVAL);
     buf_size = s->block_align;
 
     samples = data;