xfs: fix possible NULL dereference in xlog_verify_iclog

commit 643f7c4e upstream.

In xlog_verify_iclog a debug check of the incore log buffers prints an
error if icptr is null and then goes on to dereference the pointer
regardless.  Convert this to an assert so that the intention is clear.
This was reported by Coverty.

Signed-off-by: Ben Myers <bpm@sgi.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>

Signed-off-by: Jiri Slaby <jslaby@suse.cz>