Fix compilation warnings

In member function ‘void* Rocket::Core::FontDatabase::LoadFace(const String&)’:
Source/Core/FontDatabase.cpp:257:10: warning: converting ‘false’ to pointer type ‘void*’

In member function ‘bool Rocket::Core::TextureResource::Load(Rocket::Core::RenderInterface*) const’:
Source/Core/TextureResource.cpp:165:70: warning: passing NULL to non-pointer argument 1 of ‘std::pair'
Source/Core/TextureResource.cpp:177:68: warning: passing NULL to non-pointer argument 1 of ‘std::pair'