cli: Validate cli_file_index on quit command
cli: Validate cli_file_index on quit command

VPP crash happens with 'vppctl quit quit' command. The 2nd quit command
tries to access the file index which is already freed by the first quit.
This can be avoided to validate cli_file_index.

Change-Id: I880514c93523db2a727d7510c97950582cd6a6c8
Signed-off-by: Steve Shin <491df9b5284ac2b453f2c8add402bc5aa7e615e9@cisco.com>

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