api: fix crash when cf removed
api: fix crash when cf removed

cf may be removed when:
1. linux_epoll_input_inline process two EPOLLIN events, firstly a normal
message, secondly reading 0 bytes because of socket client crash, then
cf removed without clear message added to pending event data vectors
before
2. clib_file_write called

Type: fix

Signed-off-by: wanghanlin <2a7ca241eb4a7ff06d4307cab23ef63455935ec6@corp.netease.com>
Change-Id: I4523e9bb322e98357575925f3113f710d70dd679

chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp