#612 Build fails in clang-6

[Yuri]

I'm getting these warnings and erros on FreeBSD 12:

/wrkdirs/usr/ports/math/freemat/work/FreeMat-4.2-Source/libs/libMatC/CArray.cpp:125:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
/wrkdirs/usr/ports/math/freemat/work/FreeMat-4.2-Source/libs/libMatC/CArray.cpp:208:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
/wrkdirs/usr/ports/math/freemat/work/FreeMat-4.2-Source/libs/libMatC/CArray.cpp:234:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
/wrkdirs/usr/ports/math/freemat/work/FreeMat-4.2-Source/libs/libMatC/CArray.cpp:537:21: error: cannot initialize return object of type 'double' with an rvalue of type 'nullptr_t'
  if (*flag) return NULL;
                    ^~~~
/usr/include/sys/_null.h:37:14: note: expanded from macro 'NULL'
#define NULL    nullptr
                ^~~~~~~
/wrkdirs/usr/ports/math/freemat/work/FreeMat-4.2-Source/libs/libMatC/CArray.cpp:572:21: error: cannot initialize return object of type 'double' with an rvalue of type 'nullptr_t'
  if (*flag) return NULL;
                    ^~~~
/usr/include/sys/_null.h:37:14: note: expanded from macro 'NULL'
#define NULL    nullptr
                ^~~~~~~
3 warnings and 2 errors generated.