Strike null-pointer-arithmetic error in BUFFER_OFFSET

Compiling SRB2 under Xcode 11.5 complains about BUFFER_OFFSET, as seen in this build log:

[ 97%] Building C object src/sdl/CMakeFiles/SRB2SDL2.dir/__/hardware/r_opengl/r_opengl.c.o
/Users/travis/build/mazmazz/SRB2/src/hardware/r_opengl/r_opengl.c:2083:54: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
                                pglVertexPointer(3, GL_SHORT, sizeof(vbotiny_t), BUFFER_OFFSET(0));
                                                                                 ^~~~~~~~~~~~~~~~
/Users/travis/build/mazmazz/SRB2/src/hardware/r_opengl/r_opengl.c:1927:39: note: expanded from macro 'BUFFER_OFFSET'
#define BUFFER_OFFSET(i) ((char*)NULL + (i))
                          ~~~~~~~~~~~ ^

Monster Iestyn discovered an alternate define for this variable, seen here and used in this MR.

This compiles successfully and no crashes occur in runtime, as far as I've tested.