tcp: fix persist assert
tcp: fix persist assert

Type: fix

Persist and retransmit can pop at the same time.

Signed-off-by: Florin Coras <c9715e5cdf0718c2e9b10678757dfb027875eba2@cisco.com>
Change-Id: Ia9530645cc84c83a881a75d7b4627197dc50ed29

FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp