Fix a bug in subscribers counting

To avoid as much as possible allocation/deallocation
when possible sequences of subscribe/unsubscribe are
encountered, the watch strucutre of events is destroyed
only when either the event or the client is destroyed.

This commit fixes a bad handling in the count of
active subscribers.

Change-Id: I3f240e69c3b21356ff07f584fe9bfb578d46732b
Signed-off-by: José Bollo <jose.bollo@iot.bzh>