enter_chroot: skip ssh agent dir if it is invalid
enter_chroot: skip ssh agent dir if it is invalid

On long running systems, the ssh-agent dir might go stale.  This should
not prevent entering the cros chroot though, so skip it if it's invalid.

BUG=None
TEST=`cros_sdk` enters with ssh-agent dir bind mounted
TEST=`rm -rf /tmp/ssh-*/; cros_sdk` enters with ssh-agent dir skipped

Change-Id: I923b031612c0d37a896437c7355cac6c448eef82
Reviewed-on: https://gerrit.chromium.org/gerrit/12758
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

sigma/coreos-scripts,nicescale/scripts,trnubo/scripts,smilart/scripts,zhang0137/scripts,coreos/scripts,marineam/coreos-scripts,marineam/coreos-scripts,trnubo/scripts,sigma/coreos-scripts,andrejro/coreos--scripts,gtank/scripts,zhang0137/scripts,crawford/scripts,fivethreeo/scripts,vmware/coreos-scripts,BugRoger/scripts,endocode/scripts,bcwaldon/coreos-scripts,BugRoger/scripts,crawford/scripts,gtank/scripts,mjg59/scripts,fivethreeo/scripts,glevand/coreos--scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,glevand/coreos--scripts,sigma/coreos-scripts,endocode/scripts,mjg59/scripts,vmware/coreos-scripts,mischief/scripts,smilart/scripts,BugRoger/scripts,mischief/scripts,mjg59/scripts,zhang0137/scripts,endocode/scripts,crawford/scripts,mischief/scripts,nicescale/scripts,trnubo/scripts,bcwaldon/coreos-scripts,gtank/scripts,marineam/coreos-scripts,smilart/scripts,andrejro/coreos--scripts,nicescale/scripts,fivethreeo/scripts,coreos/scripts,andrejro/coreos--scripts