fix for building with latest 32 bit tools. (nw)

src/mess/video/rm380z.c: In member function 'void rm380z_state::put_point(int, int, int, int)': src/mess/video/rm380z.c:17:2: error: assuming signed overflow does not occur when assuming that (X + c) >= X is always true [-Werror=strict-overflow]
  for (int r=y;r<(y+mx);r++)
  ^
cc1plus.exe: all warnings being treated as errors
makefile:922: recipe for target 'obj/windows/mess/video/rm380z.o' failed