Ocfs2: Handle empty list in lockres_seq_start() for dlmdebug.c

This patch tries to handle the case in which list 'dlm->tracking_list' is
empty, to avoid accessing an invalid pointer. It fixes the following oops:

http://oss.oracle.com/bugzilla/show_bug.cgi?id=1287



Signed-off-by: Tristan Ye <tristan.ye@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>