nscd: Release read lock after resetting timeout.

	[BZ #22161]
	* nscd/netgroupcache.c (addinnetgrX): Release read lock after
	resetting timeout.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Andreas Schwab <schwab@suse.de>

A lock is held by mempool_allocate() when CACHEABLE is true; we
must release this lock if we exit early.