Fix compilation warnings

Source/Core/FontDatabase.cpp: In member function ‘void* Rocket::Core::FontDatabase::LoadFace(const String&)’:
Source/Core/FontDatabase.cpp:257:10: warning: converting ‘false’ to pointer type ‘void*’ [-Wconversion-null]
   return false;

Source/Core/TextureResource.cpp: 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<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = long long unsigned int; _T2 = Rocket::Core::Vector2<int>]’ [-Wconversion-null]
     texture_data[render_interface] = TextureData(NULL, Vector2i(0, 0));
                                                                      ^
Source/Core/TextureResource.cpp:177:68: warning: passing NULL to non-pointer argument 1 of ‘std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = long long unsigned int; _T2 = Rocket::Core::Vector2<int>]’ [-Wconversion-null]
   texture_data[render_interface] = TextureData(NULL, Vector2i(0, 0));
Merge pull request #106 from viciious/warnings_fix

Fix compilation warnings