pgsql: Use an always-there test, not an Assert, to check for overrun of

Log Message:
-----------
Use an always-there test, not an Assert, to check for overrun of
the held_lwlocks[] array. Per Qingqing Zhou.

Modified Files:
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pgsql/src/backend/storage/lmgr:
lwlock.c (r1.25 -> r1.26)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lwlock.c.diff?r1=1.25&r2=1.26)