client: don't set other if lookup fails in rename

On rename, only set the other inode if the
lookup for the destination succeeds, otherwise we hit
a segv in set_other_inode().

Fixes #4517.
Signed-off-by: Sam Lang <sam.lang@inktank.com>
Tested-by: Noah Watkins <jayhawk@cs.ucsc.edu>
Merge branch 'next'
Merge git://github.com/ceph/ceph