avformat/rmdec: check tag_size

Fixes: signed integer overflow: -2147483648 - 8 cannot be represented in type 'int'
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_RM_fuzzer-6598073725353984

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/rmdec.c

@@ -563,6 +563,8 @@ static int rm_read_header(AVFormatContext *s)
     }
 
     tag_size = avio_rb32(pb);
+    if (tag_size < 0)
+        return AVERROR_INVALIDDATA;
     avio_skip(pb, tag_size - 8);
 
     for(;;) {