core: Add NULL check for inode in fd_lookup
core: Add NULL check for inode in fd_lookup

Signed-off-by: Shehjar Tikoo <shehjart@gluster.com>
Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 597 (miscellaneous fixes for xlators to work well with NFS xlator)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=597

Kaushikbv/Gluster,Kaushikbv/Gluster,Kaushikbv/Gluster