wmaprodec: return an error, not 0, when the input is too small.

Returning 0 may result in an infinite loop in valid calling programs. A
decoder should never return 0 without producing any output.

CC:libav-stable@libav.org

libavcodec/wmaprodec.c

@@ -1502,8 +1502,11 @@ static int decode_packet(AVCodecContext *avctx, void *data,
         s->packet_done = 0;
 
         /** sanity check for the buffer length */
-        if (buf_size < avctx->block_align)
-            return 0;
+        if (buf_size < avctx->block_align) {
+            av_log(avctx, AV_LOG_ERROR, "Input packet too small (%d < %d)\n",
+                   buf_size, avctx->block_align);
+            return AVERROR_INVALIDDATA;
+        }
 
         s->next_packet_start = buf_size - avctx->block_align;
         buf_size = avctx->block_align;