fix HandTracker::readFrame crash

The HandTracker::readFrame works well during the lifetime of the first subscriber to /camera/gestures. Nevertheless, when this subscriber shutsdown and a new subscriber connects the first call to HandTracker::readFrame crashes.

It seems that it is not possible to robustly stop and re-start the HandTracker. Work-around implemented trying to minimise the CPU consumption when there are no subscribers
Merge branch 'hydro-devel' of github.com:jordi-pages/openni2_camera into hydro-devel