api: Avoid a NULL deref in glfs_open().

Change-Id: I23e9fabcb8a430bcd44595ca31460c54d7bd4a39
BUG: 953694
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/5426
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>