api: Add null check for active_subvol in glfs_close.

Problem:
In glfs_close, NULL value returned by glfs_active_subvol was not
being checked and was causing a crash.

Change-Id: I6eefa60e7b07dee251b98932b1d08a5c2981d3d7
BUG: 1002511
Signed-off-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-on: http://review.gluster.org/5876
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>