avcodec/diracdec: Check perspective_exp and zrs_exp.

Fixes: undefined shift
Fixes: runtime error: shift exponent 264 is too large for 32-bit type 'int'
Fixes: 2860/clusterfuzz-testcase-minimized-4672811689836544

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