Remove broken assert. ebml_read_element_id can handle level_up set to NULL.

Originally committed as revision 9621 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -310,8 +310,6 @@ ebml_peek_id (MatroskaDemuxContext *matroska,
 {
     uint32_t id;
 
-    assert(level_up != NULL);
-
     if (ebml_read_element_id(matroska, &id, level_up) < 0)
         return 0;