Fix undefined behavior on destroying an event context

[jyrkive]

Quoting the corresponding messages in IRC:

[19:49:36] FYI: the game crashed when I tried to open the Preferences dialog ingame. Looks related to the event context changes.
[19:49:37] https://gist.github.com/jyrkive/3992782bfb0c7b6bea6f9dd2ac6b9dad
[19:49:42] I'm investigating.
[20:10:19] :o
[20:21:54] * stikonas (~gentoo@wesnoth/translator/stikonas) has quit IRC (Quit: Konversation terminated!)
[20:30:50] All right, the problem is in the destructor of the context class.
[20:30:51]

wesnoth/src/events.cpp

Lines 133 to 146 in 681322b

	context::~context()
	{
	if (!handlers.empty()) {
	for (handler_list::iterator it = handlers.begin(); it != handlers.end(); ++it) {
	if ((*it)->has_joined()) {
	(*it)->has_joined_ = false;
	}
	if ((*it)->has_joined_global()) {
	(*it)->has_joined_global_ = false;
	}
	it = handlers.erase(it);
	}
	}
	}

[20:31:43] It performs "it = handlers.erase(it);" followed by "++it" in every iteration. It other words, it increments it twice.
[20:32:24] If the number of handlers happens to be odd, the second increment increments the end iterator, which is undefined behavior.
[20:32:46] Microsoft's debug CRT is guaranteed to crash in that situation, fortunately.
[20:32:46] I think logically the ++it is not supposed to be there.
[20:33:14] It doesn't make sense to manually remove the handlers from the list anyway.
[20:33:16] Yeah, MSVC tends to be pretty good about crashing in these sorts of situations.
[20:33:29] Hmm.
[20:33:42] This is the destructor of the context class. The destructor of the list is just about to be executed, and that would clear the list for us.