Audio disappears if Process is killed

[panagiac]

Hi!

I think I found a way to replicate the issue partially described in #22

If you start a Call, and then kill the process while it is still in Call, the next instance of the liblinphone app will be muted.

Using Process.Kill() during a Call, I can replicate this issue almost every time. After some while, audio will be back working.

Thank you