MSVC: Fix some warnings in C wrapper.
MSVC: Fix some warnings in C wrapper.

Implicit casts to bool -> warning.

KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross