Fix memory leak when ClassDB::bind_method_custom() fails

[dsnopek]

Fixes #101870

In other cases (for example, ClassDB::_bind_vararg_method() and ClassDB::bind_methodfi()), the method bind is deleted just before returning if it can't be added for some reason.

A couple examples:

godot/core/object/class_db.cpp

Lines 1899 to 1902 in 71d80b2

	if (!type) {
	memdelete(bind);
	ERR_FAIL_NULL_V(type, nullptr);
	}

godot/core/object/class_db.cpp

Lines 1949 to 1953 in 71d80b2

	if (!p_compatibility && type->method_map.has(mdname)) {
	memdelete(p_bind);
	// overloading not supported
	ERR_FAIL_V_MSG(nullptr, vformat("Method already bound '%s::%s'.", instance_type, mdname));
	}

So, this PR is aiming to do the same thing but in ClassDB::_bind_method_custom().

(Because I needed to stash p_method->get_name() in a variable so I could use it after the method bind is deleted, I also did a tiny refactor to stash that in a local variable for all cases, since it was referred to quite a lot.)

[RandomShaper]

Name stashing looks like needed here, too.

[dsnopek]

Sorry, it looks like I didn't push at the end? Should be right now.

[Repiteo]

Thanks!