avcodec/sheervideo: Check input buffer size before allocating and decoding

Fixes: Timeout
Fixes: 1858/clusterfuzz-testcase-minimized-6450473802399744

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