Plug route leak in IP output.

Plug route leak in IP output.

If no struct route is passed to ip_output() or ip6_output(), it
uses its own iproute on the stack.  In that case any route entry
in the local route cache has to be freed.  After pf decides to
reroute, struct route is reset to NULL.  Then the route reference
counter has to be released.  Call rtfree() without needless NULL
check.

OK mvs@