Revert "Disallow units with negative HP"

This reverts commit ef60dea.

@ln-zookeeper pointed out that units with negative HP are documented in
the wiki, and thus disallowing them is an API
change.

src/scripting/lua_unit.cpp

@@ -391,6 +391,7 @@ static int impl_unit_set(lua_State *L)
 	//modify_int_attrib_check_range("side", u.set_side(value), 1, static_cast<int>(teams().size())); TODO: Figure out if this is a good idea, to refer to teams() and make this depend on having a gamestate
 	modify_int_attrib("side", u.set_side(value));
 	modify_int_attrib("moves", u.set_movement(value));
+	modify_int_attrib("hitpoints", u.set_hitpoints(value));
 	modify_int_attrib("experience", u.set_experience(value));
 	modify_int_attrib("recall_cost", u.set_recall_cost(value));
 	modify_int_attrib("attacks_left", u.set_attacks(value));
@@ -405,19 +406,6 @@ static int impl_unit_set(lua_State *L)
 
 	modify_vector_string_attrib("extra_recruit", u.set_recruits(value));
 	modify_vector_string_attrib("advances_to", u.set_advances_to(value));
-
-	if(strcmp(m, "hitpoints") == 0) {
-		int hp = luaL_checkinteger(L, 3);
-		if(hp >= 0) {
-			u.set_hitpoints(hp);
-			return 0;
-		} else {
-			std::string err_msg = "invalid target HP: ";
-			err_msg += std::to_string(hp);
-			return luaL_argerror(L, 3, err_msg.c_str());
-		}
-	}
-
 	if(strcmp(m, "alignment") == 0) {
 		u.set_alignment(lua_check<unit_type::ALIGNMENT>(L, 3));
 		return 0;

src/units/unit.cpp

@@ -50,7 +50,6 @@
 #include "units/map.hpp"	   // for unit_map, etc
 #include "variable.hpp"		   // for vconfig, etc
 #include "version.hpp"
-#include "wml_exception.hpp"
 
 #include "utils/functional.hpp"
 #include <boost/dynamic_bitset.hpp>
@@ -630,7 +629,6 @@ 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"));
 		hit_points_ = *v;
 	} else {
 		hit_points_ = max_hit_points_;