afr: check for non-zero call_count before doing a stack wind

When one of the bricks of a 1x2 replicate volume is down,
writes to the volume is causing a race between afr_flush_wrapper() and
afr_flush_cbk(). The latter frees up the call_frame's local variables
in the unwind, while the former accesses them in the for loop and
sending a stack wind the second time. This causes the FUSE mount process
(glusterfs) toa receive a SIGSEGV when the corresponding unwind is hit.

This patch adds the call_count check which was removed when
afr_flush_wrapper() was introduced in commit 29619b4e

Change-Id: I87d12ef39ea61cc4c8244c7f895b7492b90a7042
BUG: 988182
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/5393
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>