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>
(cherry picked from commit 2cb7ee8a)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>