pgsql: If we're going to have a non-panic check for held_lwlocks[]

Log Message:
-----------
If we're going to have a non-panic check for held_lwlocks[] overrun,
it must occur *before* we get into the critical state of holding a
lock we have no place to record. Per discussion with Qingqing Zhou.

Modified Files:
--------------
pgsql/src/backend/storage/lmgr:
lwlock.c (r1.26 -> r1.27)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lwlock.c.diff?r1=1.26&r2=1.27)