Fix last argument in calls to dCamera_c::startEventCamera

The last argument is used as a char*, so we need to explicitly
pass nullptr instead of 0 to ensure it has the correct type.
This was causing crashes due to va_arg reading 8 bytes when only 4
were provided by the caller - if the out-of-bounds bytes are
non-zero, the null check fails and it continues pulling args from
beyond the argument buffer potentially triggering a segfault.

src/d/actor/d_a_obj_tks.cpp

@@ -164,7 +164,7 @@ int daObjTks_c::Create() {
         field_0xde1 = 1;
 
         static const int m_cameraItemNum = 1;
-        dCam_getBody()->StartEventCamera(18, fopAcM_GetID(this), "Type", 1, &m_cameraItemNum, 0);
+        dCam_getBody()->StartEventCamera(18, fopAcM_GetID(this), "Type", 1, &m_cameraItemNum, nullptr);
     }
 
     return phase_state;