math/saga: fix build on GCC architectures
Returning NULL as bool is not ok.
- globe_gores.lo ---
globe_gores.cpp: In member function 'bool CGlobe_Gores::Add_Gore(int, int)':
globe_gores.cpp:210:16: error: converting to 'bool' from 'std::nullptr_t' requires direct-initialization [-fpermissive]
210 | return( NULL );
PR: 246720
Approved by: rhurlin@gwdg.de (maintainer)