threads: add assert to silence coverity warning
threads: add assert to silence coverity warning

Change-Id: I381786b26050f2dfcb4c74094309cf2114d26d85
Signed-off-by: Damjan Marion <9141bba8b2efed526e55cf796d48631af330ad98@cisco.com>

chrisy/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,GabrielGanne/vpp-flowtable,vpp-dev/vpp,milanlenco/vpp,FDio/vpp,GabrielGanne/vpp-flowtable,FDio/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,milanlenco/vpp,GabrielGanne/vpp-flowtable,FDio/vpp,milanlenco/vpp,chrisy/vpp,vpp-dev/vpp,GabrielGanne/vpp-flowtable,chrisy/vpp,vpp-dev/vpp,GabrielGanne/vpp-flowtable,FDio/vpp,milanlenco/vpp,chrisy/vpp,FDio/vpp,milanlenco/vpp,milanlenco/vpp,GabrielGanne/vpp-flowtable,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp