Fix incorrect check for nullptr that triggers GCC 11 warning
- This method seems to be unused, so anyway not affecting anything
Fixes this warning with GCC 11:
/solarus/src/core/Map.cpp: In member function ‘void Solarus::Map::build_foreground_surface()’:
/solarus/src/core/Map.cpp:605:45: warning: ‘this’ pointer is null [-Wnonnull]
605 | const Size& camera_size = camera->get_size();
| ~~~~~~~~~~~~~~~~^~