Fix leak of attack types

When I first implemented this I did not fully understand the contract of boost::intrusive_ptr.

src/units/attack_type.hpp

@@ -133,7 +133,9 @@ class attack_type
 	}
 
 	friend void intrusive_ptr_release(const attack_type* atk) {
-		--atk->ref_count;
+		if(--atk->ref_count == 0) {
+			delete atk;
+		}
 	}
 };