Return failure in getnetgrent only when all netgroups have been searched (#17363)

The netgroups lookup code fails when one of the groups in the search
tree is empty.  In such a case it only returns the leaves of the tree
after the blank netgroup.  This is because the line parser returns a
NOTFOUND status when the netgroup exists but is empty.  The
__getnetgrent_internal implementation needs to be fixed to try
remaining groups if the current group is entry.  This patch implements
this fix.  Tested on x86_64.

	[BZ #17363]
	* inet/getnetgrent_r.c (__internal_getnetgrent_r): Try next
	group if the current group is empty.