[bluez] Fix freed pointer reference. Fixes JB#10268.

It could happen that a callback function would request an abort when
stream state was changing to idle and the related structure would be
freed before abort response would arrive. Quick hack to prevent a
crash.
Merge pull request #17 from hmallat/AVDTP-stream-abort-handling

[bluez] Fix freed pointer reference. Fixes JB#10268.