Return and warn if a handle's refcount is <= 0
Return and warn if a handle's refcount is <= 0

This makes it more likely consumers notice invalid unreferences.
GObject has the same assertion.

http://bugzilla.openedhand.com/show_bug.cgi?id=2029

Signed-off-by: Emmanuele Bassi <049ee2061f71220e5aaf33f5be422fc2a2b06a44@linux.intel.com>

djdeath/cogl-android,djdeath/cogl-android,djdeath/cogl-android,Distrotech/cogl,Distrotech/cogl,gcampax/cogl,collects/cogl,gcampax/cogl,Distrotech/cogl,gcampax/cogl,djdeath/cogl,collects/cogl,gcampax/cogl,djdeath/cogl,spatulasnout/cogl,djdeath/cogl,spatulasnout/cogl,Distrotech/cogl