Avoid prematurely destroying callback_rxcon

Avoid prematurely destroying callback_rxcon

Currently, h_GetHost_r and removeAddress_r can destroy the
callback_rxcon of a host. Having a NULL callback_rxcon can cause
segfaults in code that does not properly check if a host has been
HOSTDELETED before trying to use it.

Although such code is incorrect and should be fixed, we can still avoid
a segfault in those situations by not destroying callback_rxcon until we
destroy the host itself. This just prevents destroying callback_rxcon in
h_GetHost_r and removeAddress_r, leaving it to h_TossStuff_r to destroy
when it destroys the host.

Reviewed-on: http://gerrit.openafs.org/717
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit bbcfbe1a04eda9e75b1643be88cf9d4842a8aa86)

Change-Id: I4d44fca9b79b656bc70f8108616f23f2dfaa353f
Reviewed-on: http://gerrit.openafs.org/749
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>