avcodec/rv34: Fix runtime error: signed integer overflow: 36880 * 66288 cannot be represented in type 'int'

Fixes: 768/clusterfuzz-testcase-4807444305805312

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