Codechange: add non-nullptr asserts in cases where it should never be nullptr

Though where similar calls are checked for nullptr as in those instances of
the use of that function it can actually return nullptr. In other words, write
down the assumption that the function never returns nullptr in an assert.

src/newgrf_engine.cpp

@@ -518,6 +518,7 @@ static uint32 VehicleGetVariable(Vehicle *v, const VehicleScopeResolver *object,
 
 			{
 				const Vehicle *w = v->Next();
+				assert(w != nullptr);
 				uint16 altitude = ClampToU16(v->z_pos - w->z_pos); // Aircraft height - shadow height
 				byte airporttype = ATP_TTDP_LARGE;