[core] Fix crash when constructing an air absorption model with a callback

[wolframw]

fixes #519

[MaxenceMaire]

Just tested this fix and can confirm it resolves the segfault.

My minimal test case from #519 now passes:

Starting test...
Context create returned: 0
About to call iplAirAbsorptionCalculate...
Callback called: distance=10.000000, band=0
Callback called: distance=10.000000, band=1
Callback called: distance=10.000000, band=2
Success! Results: 0.000000, 0.000000, 0.000000

The callback is being invoked correctly and no more crashes. I initially thought this was a C struct initialization issue on my end, but turns out the constructor was unconditionally dereferencing the coefficients pointer even when it might not be valid during the C-to-C++ conversion.