MFC r299513,r299515:

MFC r299513,r299515:

r299513 (by cem):

rtadvd(8): Don't use-after-free

This whole block of code as committed fully formed in r224144.  I'm not really
sure what the intent was, but it seems plausible that !persist ifis could need
other member cleanup.  Don't free the object until after we've finished
cleaning its members.

CID:		1006079

r299515 (by cem):

rtadvd(8): Fix use-after-close in cm_handler_client

cm_send() closes 'fd' on error.  In that case, bail out early without trying to
recv from or close 'fd' again.

CID:		1006078

git-svn-id: svn://svn.freebsd.org/base/stable/10@301809 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f