Clear 'ia' after iterating if_addrhead for unicast address matching: since

Clear 'ia' after iterating if_addrhead for unicast address matching: since
'ifa' was used as the TAILQ_FOREACH() iterator argument, and 'ia' was just
derived form it, it could be left non-NULL which confused later
conditional freeing code.  This could cause kernel panics if multicast IP
packets were received.  [1]

Call 'struct in_ifaddr *' in ip_rtaddr() 'ia', not 'ifa' in keeping with
normal conventions.

When 'ipstealth' is enabled returns from ip_input early, properly release
the 'ia' reference.

Reported by:	lstewart, sam [1]
MFC after:	6 weeks