lua: Don't check_stack in LuaCheckStack destructor

This would cause an assertion failure in functions like lua_for_each
when the given closure throws an error and thus leaves some values on
the stack. This can make Aegisub crash entirely instead of just catching
and reporting the error. Instead, these stack_checks can be done
manually.

libaegisub/include/libaegisub/lua/utils.h

@@ -124,7 +124,6 @@ struct LuaStackcheck {
 	void dump();
 
 	LuaStackcheck(lua_State *L) : L(L), startstack(lua_gettop(L)) { }
-	~LuaStackcheck() { check_stack(0); }
 };
 #else
 struct LuaStackcheck {

src/auto4_lua.cpp

@@ -517,6 +517,7 @@ namespace {
 		if (lua_isnumber(L, -1) && lua_tointeger(L, -1) == 3) {
 			lua_pop(L, 1); // just to avoid tripping the stackcheck in debug
 			description = "Attempted to load an Automation 3 script as an Automation 4 Lua script. Automation 3 is no longer supported.";
+			stackcheck.check_stack(0);
 			return;
 		}
 
@@ -529,6 +530,7 @@ namespace {
 			name = GetPrettyFilename().string();
 
 		lua_pop(L, 1);
+		stackcheck.check_stack(0);
 		// if we got this far, the script should be ready
 		loaded = true;
 	}