glusterd: fix segfault on volume status detail

If for some reason glusterd_get_brick_root() fails,
it frees the gf_strdup'ed *mount_point in its own error path,
and returns -1.

Unfortunately it already had assigned that pointer value
to the output argument, the caller function
glusterd_add_brick_detail() sees a non-NULL pointer,
and free() again: segfault.

Could be fixed with a one-liner (*mount_point = NULL)
in the error path, but I think glusterd_get_brick_root()
should only assign to the output argument once all checks passed,
so I use a local temporary pointer, which increases the patch a bit.

Change-Id: I3f3035f01e80a5e9bdf2da895e4cf7baa3dfbd2f
BUG: 919352
Signed-off-by: Lars Ellenberg <lars@linbit.com>
Reviewed-on: http://review.gluster.org/4646
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-on: http://review.gluster.org/4841
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>