Harden unit_movement_resetter class.

Add a warning regarding its usage and test whether the unit exists
before assigning to it. (Still a bit experimental, but initial tests
show no regressions.)

src/unit.cpp

@@ -2799,6 +2799,14 @@ unit_movement_resetter::unit_movement_resetter(unit &u, bool operate) :
 
 unit_movement_resetter::~unit_movement_resetter()
 {
+	assert(resources::units);
+
+	/*
+	 * This assert should be safe, but not 100% sure so if it fails need
+	 * some extra validation code in this function.
+	 */
+	assert(resources::units->has_unit(&u_));
+
 	u_.movement_ = moves_;
 }
 

src/unit.hpp

@@ -467,8 +467,16 @@ private:
 	void clear_visibility_cache() const { invisibility_cache_.clear(); }
 };
 
-/** Object which temporarily resets a unit's movement */
+/**
+ * Object which temporarily resets a unit's movement.
+ *
+ * @warning
+ * The unit whose movement is reset may not be deleted while a
+ * @ref unit_movement_resetter object 'holds'. So best use it only in a small
+ * scope.
+ */
 struct unit_movement_resetter
+	: private boost::noncopyable
 {
 	unit_movement_resetter(unit& u, bool operate=true);
 	~unit_movement_resetter();