Cellular: fixed null pointer bug in cellular fsm

[jarvte]

Description

Cellular: fixed possible crash bug in cellular state machine.
After stop() call there might have been a call to null pointer.

@AriParkkila please review

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

How does it crash and how is it fixing? (add some detailed info to the commit message if possible)

[jarvte]

@0xc0170 updated commit and pr message

[0xc0170]

@0xc0170 updated commit and pr message

I see the title changed (the title should be 50 characters the most, same as first line in the commit message. The title here is used also in the release notes). The commit message (1st paragraph should contain the details about it). but commit has not changed, a push missing?

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1816
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6677/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1462
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6677/

[mbed-ci]

Test : SUCCESS

Build number : 1624
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6677/1624

[jarvte]

@0xc0170 merge?