Don't construct an intrusive pointer from a raw pointer

@gfgtdf is worried about the possibility of this code being called from a unit's constructor, which would create problems.

src/units/abilities.cpp

@@ -45,7 +45,7 @@ namespace {
 	class temporary_facing
 	{
 		map_location::DIRECTION save_dir_;
-		unit_const_ptr u_;
+		const unit* u_;
 	public:
 		temporary_facing(const unit* u, map_location::DIRECTION new_dir)
 			: save_dir_(u ? u->facing() : map_location::NDIRECTIONS)
@@ -949,8 +949,8 @@ bool attack_type::special_active(const config& special, AFFECTS whom,
 	assert(display::get_singleton());
 	const unit_map& units = display::get_singleton()->get_units();
 
-	const unit* self = self_.get();
-	const unit* other = other_.get();
+	const unit* self = self_;
+	const unit* other = other_;
 
 	if(self == nullptr) {
 		unit_map::const_iterator it = units.find(self_loc_);

src/units/attack_type.hpp

@@ -109,7 +109,8 @@ class attack_type : public std::enable_shared_from_this<attack_type>
 	// Used via set_specials_context() to control which specials are
 	// considered active.
 	mutable map_location self_loc_, other_loc_;
-	mutable unit_const_ptr self_, other_;
+	mutable const unit* self_;
+	mutable const unit* other_;
 	mutable bool is_attacker_;
 	mutable const_attack_ptr other_attack_;
 	mutable bool is_for_listing_ = false;