dng spec 1.5.0.0
"BitsPerSample
Supported values are from 8 to 32 bits/sample. The depth must be the same for each sample if
SamplesPerPixel is not equal to 1."

Fixes: eg_crash
Found-by: 黄宁 <tsukimurarin@163.com>
Reviewed-by: Nick Renieris <velocityra@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>