Fix: sessiond: off-by-one poll check when draining an event notifier

When an event source is removed (on the death of an application), the
notification thread "drains" any remaining notifications from the
event notifier pipe.

In doing so, it creates a new poll set containing the event source to
check if messages are left in the event notification pipe.

The invocation of `LTTNG_POLL_GETEV(&events, 1)` means to check the
events pending for the first (and only) fd in the poll set. This check
is off by one since `0` should be used.

For some reason, this worked everywhere except when using a 32-bit
userland on a 64-bit kernel (on x86_64).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6f274fdd7c80d5676fd48ae20a14adb3cc010142