cluster/afr: Avoid Null de-reference when post parent is NULL
cluster/afr: Avoid Null de-reference when post parent is NULL

Signed-off-by: Pranith Kumar K <pranithk@gluster.com>
Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 857 (Crash in afr_sh_entry_expunge_entry_cbk)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=857

Kaushikbv/Gluster,Kaushikbv/Gluster,Kaushikbv/Gluster