Fix load_string() ignoring the passed chunk name (#6794)

Complete with a unit test!
The test results in PASS_BY_VICTORY if it fails,
which is a bit weird, but it does work.

data/test/scenarios/test_lua_name.cfg

@@ -0,0 +1,18 @@
+
+# Test that an error message in Lua includes the value of the tag's name key
+{GENERIC_UNIT_TEST test_lua_name (
+    [event]
+        name=start
+        [lua]
+            name=test_lua_name
+            code=<<
+                -- Can't use assert_throws_with here because we want to test the preamble that's automatically prefixed to the error.
+                local ret, err = pcall(function()
+                    local some_variable = bar
+                end)
+                unit_test.assert_contains(err, 'test_lua_name', 'error contains name')
+                unit_test.succeed()
+            >>
+        [/lua]
+    [/event]
+)}

src/scripting/lua_kernel_base.cpp

@@ -1021,7 +1021,7 @@ bool lua_kernel_base::protected_call(lua_State * L, int nArgs, int nRets, error_
 bool lua_kernel_base::load_string(char const * prog, const std::string& name, error_handler e_h)
 {
 	// pass 't' to prevent loading bytecode which is unsafe and can be used to escape the sandbox.
-	int errcode = luaL_loadbufferx(mState, prog, strlen(prog), name.empty() ? name.c_str() : prog, "t");
+	int errcode = luaL_loadbufferx(mState, prog, strlen(prog), name.empty() ? prog : name.c_str(), "t");
 	if (errcode != LUA_OK) {
 		char const * msg = lua_tostring(mState, -1);
 		std::string message = msg ? msg : "null string";

wml_test_schedule

@@ -139,6 +139,7 @@
 0 test_ability_id_active
 0 test_ability_id_not_active
 0 event_test_filter_attack
+0 test_lua_name
 0 filter_vision
 0 test_shroud_init
 0 test_shroud_place_wml