Fix the crash on linux

src/Unit.cpp

@@ -341,7 +341,7 @@ Unit Unit::getTarget() const
 		CCUnitID t_id = getUnitPtr()->orders[0].target_unit_tag;
 
 		// if it doesn't have a target. Return itself
-		if (m_bot->GetUnit(t_id) == nullptr) {
+		if (m_bot->GetUnit(t_id).isValid()) {
 			return *this;
 		}