mgcp: be sure that pending mgcp transactions are canceled before free

When the FSM reaches ST_HALT it frees itsself and all context
information but it is not ensured that there are still pending
MGW transactions that might hit late and eventually cause a use after
free situation.

- if an MGW transaction is still pending, cancel it.

Change-Id: I8ff55e48a95cc4c556a97ad2593bad1cc1aa69bd
1 file changed