fuse: fix oops in revalidate when called with NULL nameidata

Some cases (e.g. ecryptfs) can call ->dentry_revalidate with NULL
nameidata.

https://bugzilla.kernel.org/show_bug.cgi?id=34732

Tyler Hicks pointed out that this bug was introduced by commit
e7c0a16786 "fuse: make fuse_dentry_revalidate() RCU aware"

Reported-by: Witold Baryluk <baryluk@smp.if.uj.edu.pl>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>