Bug 985949: Fix pointer handling in BlueZ backend, r=echou

This is caused by bug 967364. We need to hand over data structures
to DBus operations and forget about them if the operation returned
success. Until now, we could just tell their nsRefPtrs to |forget|
about them, but with bug 967364 applied we need to swap an empty
value into the pointer.
Bug 985949: Correctly forget pointer references in Bluedroid backend, r=echou

This patch fixes a regression from bug 967364. An nsRefPtr is now
correctly cleared by |forget| without causing a crash.