[obexd] pbap: clean up any possible request at session disconnect

Make sure that requests are cleaned (and associated D-Bus method calls
cancelled) on disconnect. Also added pointer clearing to the cleanup
code so that freed memory won't be considered still valid by mistake.

Unref sent D-Bus messages and pending calls as needed.
Merge pull request #20 from hmallat/pbap-request-cleanup

[obexd] pbap: clean up any possible request at session disconnect