matroskadec: fail cleanly when the required compression lib is not compiled in

Originally committed as revision 14870 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -919,6 +919,8 @@ static int matroska_decode_buffer(uint8_t** buf, int* buf_size,
         break;
     }
 #endif
+    default:
+        return -1;
     }
 
     *buf = pkt_data;