cli: fix null pointer reference if cli thread cannot be started/ended
cli: fix null pointer reference if cli thread cannot be started/ended

Signed-off-by: Ivan Koveshnikov <64dccc08362effaacf5312029450430dfca124ef@gmail.com>
Reviewed-by: Sorin Vultureanu <8013ba55f8675034bc2ab0d6c3a1c9650437ca36@enea.com>

TolikH/ofp,TolikH/ofp,OpenFastPath/ofp,TolikH/ofp,OpenFastPath/ofp,OpenFastPath/ofp,OpenFastPath/ofp