Fix crash when plugin_set_key_group() is called several times by plugins

[techee]

When plugin calls plugin_set_key_group() several times for the same
group (when creating keybindings dynamically and needs to reset them),
it crashes with the current code the second time it gets called.

The reason is that group->plugin_keys is an array into which entries of
group->key_items point and when calling

g_ptr_array_set_size(group->key_items, 0);

it calls free_key_binding() for every item - when these items are
deallocated by g_free(group->plugin_keys) previously, calls of
free_key_binding() reference an invalid memory.

Just first resizing group->key_items (and calling free_key_binding() for
its items) and freeing group->plugin_keys afterwards fixes the problem.

[kugel-]

LGTM (@b4n @elextr)

[elextr]

@kugel- have you tested this, I wouldn't even know where to start to trigger it?

[kugel-]

No, i haven't tested it. The change is trivial and @techee explaination makes complete sense. The first post also mentions how to trigger

[elextr]

@techee @kugel- the "how to trigger" is in terms of code inside a plugin, which plugin?

[kugel-]

Probably one that's currently in development (@techee mentions one on #1430)

[elextr]

Ok, so you can test it to see if it doesn't crash for other plugins.

[techee]

@elextr No other plugin calls this function several times right now - at the moment all plugins just define their keybindings in the init function and they are done. So no existing plugin crashes.

[elextr]

ok, there are two ways of looking at this:

1. no visible code uses it, and no other reports of the problem, and since @techee already has the fix there is no rush to commit this and it can be committed when @techee publishes his super-plugin

2. since this function is effectively part of the code of plugin_set_key_group() (which is the only user, so the fix cannot affect other code, by inspection), and that is in the API, the bug report should be fixed before anyone else uses it

I incline to the second view, so it nobody objects within a few days will commit it.

[elextr]

Sorry, the "few" days stretched, committed