vhost-user: fix crash when descriptor points to unknown region
vhost-user: fix crash when descriptor points to unknown region

This happens only on when compiled for older microarchitectures,
where BSF insutruction is used instead of TZCNT. BSF provides
undefined result if operand is 0.

Change-Id: I7a13350786a533428168595097ef01a560fde53b
Signed-off-by: Damjan Marion <9141bba8b2efed526e55cf796d48631af330ad98@cisco.com>

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