Allow modifying dead units in last_breath and die event handlers

Fixes an unintentional API change from commit 3c344e8, discussed in
comments of wesnoth#3042.

src/actions/attack.cpp

@@ -1291,6 +1291,8 @@ void attack::unit_killed(unit_info& attacker,
 
 	std::string undead_variation = defender.get_unit().undead_variation();
 
+	unit::dying_unit_loc = defender.loc_;
+
 	fire_event("attack_end");
 	refresh_bc();
 
@@ -1315,6 +1317,7 @@ void attack::unit_killed(unit_info& attacker,
 
 	// WML has invalidated the dying unit, abort.
 	if(!defender.valid() || defender.get_unit().hitpoints() > 0) {
+		unit::dying_unit_loc = map_location::null_location();
 		return;
 	}
 
@@ -1339,6 +1342,8 @@ void attack::unit_killed(unit_info& attacker,
 	resources::game_events->pump().fire("die", death_loc, attacker_loc, dat);
 	refresh_bc();
 
+	unit::dying_unit_loc = map_location::null_location();
+
 	if(!defender.valid() || defender.get_unit().hitpoints() > 0) {
 		// WML has invalidated the dying unit, abort
 		return;

src/units/unit.cpp

@@ -207,6 +207,8 @@ namespace
 	}
 } // end anon namespace
 
+map_location unit::dying_unit_loc;
+
 /**
  * Intrusive Pointer interface
  *
@@ -630,7 +632,9 @@ void unit::init(const config& cfg, bool use_traits, const vconfig* vcfg)
 	}
 
 	if(const config::attribute_value* v = cfg.get("hitpoints")) {
-		VALIDATE(*v > 0, _("Unit with negative HP found"));
+		if(loc_ != dying_unit_loc) {
+			VALIDATE(*v > 0, _("Unit with negative HP found"));
+		}
 		hit_points_ = *v;
 	} else {
 		hit_points_ = max_hit_points_;

src/units/unit.hpp

@@ -1598,6 +1598,15 @@ class unit
 	void set_appearance_changed(bool value) { appearance_changed_ = value; }
 	bool appearance_changed() const { return appearance_changed_; }
 
+	/**
+	 * The location of the dying unit for the duration of last_breath and die events.
+	 * Null location when neither of those events is running.
+	 * This exists in order to detect at unit creation time whether the WML/Lua code is unstoring a unit
+	 * that was already dead (e.g. as a result of [modify_unit], which is implemented with unstoring
+	 * under the hood). That's the only situation where creating a unit with negative HP is allowed.
+	 */
+	static map_location dying_unit_loc;
+
 protected:
 	mutable long ref_count_; // used by intrusive_ptr