GDALDestroyWarpOptions to do nothing for null

[Mateusz Łoskot]

GDALDestroyWarpOptions function should follow behaviour of std::free function and other standard C/C++ deallocators and should do nothing if nullptr is passed.

Currently, it throws error if psOptions is nullptr.

The change is trivial:

void CPL_STDCALL GDALDestroyWarpOptions( GDALWarpOptions *psOptions )
{
   if (psOptions)
   {
      // current definition
   }
}