avformat/matroskadec: Assert that num_levels is non negative

Maybe Closes: CID1452496 Uninitialized scalar variable

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskadec.c

@@ -4210,7 +4210,7 @@ static int matroska_parse_cluster(MatroskaDemuxContext *matroska)
     MatroskaBlock     *block = &cluster->block;
     int res;
 
-    av_assert0(matroska->num_levels <= 2);
+    av_assert0(matroska->num_levels <= 2U);
 
     if (matroska->num_levels == 1) {
         res = ebml_parse(matroska, matroska_segment, NULL);