games/iortcw: Fix build with clang 6.0

code/splines/util_str.cpp:477:11: error: reinterpret_cast from 'nullptr_t' to
'const char *' is not allowed

idStr e( reinterpret_cast<const char *>( NULL ) );
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

code/splines/util_str.cpp:491:10: error: reinterpret_cast from 'nullptr_t' to
'const char *' is not allowed

a = c + reinterpret_cast<const char *>( NULL );
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

code/splines/util_str.cpp:494:6: error: reinterpret_cast from 'nullptr_t' to
'const char *' is not allowed

a = reinterpret_cast<const char *>( NULL ) + d;
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

code/splines/util_str.cpp:498:7: error: reinterpret_cast from 'nullptr_t' to
'const char *' is not allowed

a += reinterpret_cast<const char *>( NULL );
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PR: 224988
Reported by: jbeich