avcodec/pngdec: Check bit depth for validity

Fixes: runtime error: shift exponent 132 is too large for 32-bit type 'int'
Fixes: 609/clusterfuzz-testcase-4825202619842560

See 11.2.2 IHDR Image header

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