avformat/flvdec: Use 64bit for sum_flv_tag_size

Fixes: signed integer overflow: 2138820085 + 16130322 cannot be represented in type 'int'
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_LIVE_FLV_fuzzer-6704728165187584

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>