[connman] Check for pointer validity before cancelling agent request

In the case where agent disappears from D-Bus while there are pending
requests, request->driver may become null. As it's useless to send a
Cancel to a nonexistent agent, not to mention accessing null pointer
causes a crash, check for the pointer value before creating a D-Bus
message.
Merge pull request #147 from hmallat/check-driver-before-dbus-call

[connman] Check for pointer validity before cancelling agent request