cluster/stripe: Fix EINVAL errors on quota enabled volumes

Write operations on directories with quota enabled used to fail with
EINVAL on stripe volumes. This was due to assert failure in
stripe_lookup(), meant to ensure loc->path is not NULL. However,
in nameless lookup (in this particular case triggered by quotad, which
has stripe xlator in its graph), loc->path can be legitimately NULL.

The fix involves removing this check in stripe_lookup().

Change-Id: Ibbd4f68763fdd8a85f29da78b3937cef1ee4fd1e
BUG: 1100050
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/8145
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>