agent: don't cancel a request if user already replied

avoids a crash

user want to connect:
__connman_service_connect() service 0x80f71f0 state idle
we send a request to the agent for passphrase
src/agent.c:connman_agent_queue_message() driver 0x80df2a8
src/service.c:connman_service_ref_debug() 0x80f71f0 ref 2 by src/service.c:6742:agent_context_ref()
We launch a scan
plugins/wifi.c:throw_wifi_scan() device 0x80eaf28 0x80f03d0

Our network disappear
plugins/wifi.c:network_removed() name Hello
src/service.c:connman_service_unref_debug() 0x80f71f0 ref 1 by src/service.c:6601:__connman_service_remove_from_network()

the agent return the input
src/service.c:request_input_cb() RequestInput return, 0x80f71f0

We unref agent context and the service
connman_service_unref_debug() 0x80f71f0 ref 0 by src/service.c:6749:agent_context_unref()

We cancel the request because the service is no longer here
src/agent.c:connman_agent_cancel() context 0x80f71f0

We unref again the service
src/service.c:connman_service_unref_debug() 0x80f71f0 ref -1 by src/service.c:6749:agent_context_unref()

And we segfault