ev: stop event listeners during context destroy

ev: stop event listeners during context destroy

I think I've found a bug in libev backend, in function lws_libev_io(). I'm using latest version from master branch.

When deleting a context with active connections via lws_context_destroy(), context->being_destroyed is set to 1 early in the function, before the loop calling lws_close_free_wsi() on each active connection.
lws_close_free_wsi() calls remove_wsi_socket_from_fds(), which calls lws_libev_io(), and here is my problem :

lws_libev_io() returns without doing anything if context->being_destroyed is set, so libev callbacks for deleted connections file descriptors stay registered after context is destroyed, which may lead to segfault/undefined behaviour if these file descriptors get reused later (which would trigger the callbacks).

I think the "if (!pt->io_loop_ev || context->being_destroyed) return;" statement should be replaced with " if (!pt->io_loop_ev) return;"

This fixes the problem for me and I have not seen any side effect yet. Moreover, libuv backend does not have such a test.