Fix: event notifier group: fix fd leak on error

Use a regular pattern for all commands:

If the command callback takes ownership of a pointer or file descriptor,
it sets them to NULL or -1. Therefore, the caller can always try to free
the pointer, or close it if it is greater or equal to 0.

This eliminates memory and fd leaks on error.

Change-Id: If5181a0c438f2e1ac73e5d4d63d5cde21cc97e52
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>