fix race condition at event_handles

fix race condition at event_handles

event_handles supposed just run at a thread.

There are re-entry check at begin.

1: if (usbi_handling_events(ctx))
2:	return LIBUSB_ERROR_BUSY;

3: usbi_stat_event_handle(ctx);

this code is hold any lock

it is possible two thread check 1 at the same time, then
go through to 3. So two threads will run event_handles.

above 3 line code should hold event_data_lock to avoid above
race condition.

1: usbi_mutex_lock($ctx->event_data_lock);
2: r = 0;
3: if (usbi_handling_events(ctx))
4: 	r = LIBUSB_ERROR_BUSY;
5: else
6: 	usbi_start_event_handling(ctx);
7: usbi_mutex_unlock($ctx->event_data_lock);

8: if(r)
9:	return r;

check and set in an atomic operations.

Closes #520

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Nathan Hjelm <hjelmn@me.com>