homed: Release(): fix assertion failure

homed: Release(): fix assertion failure

This fixes a race condition crash in homed that would happen in the
following sequence of events:

1. Client 1 takes a ref on the home area
2. Client 1 calls some method via dbus
3. Client 2 calls Release()

In homed, the Release() would check if a ref is still held (in this
case: yes it is) and returns an error. Except that is done through a
code-path that asserts that no operations are ongoing. In this case,
it's valid to have an ongoing operation, and so the assertion fails
causing homed to crash.