eigrpd: Fix eigrp crash on shut of a interface

eigrpd: Fix eigrp crash on shut of a interface

The eigrp interface structure was storing a pointer to the
connected interface structure and on shutdown of an interface
this would cause zebra to call eigrp back with a shutdown of
that interface, as part of that operation the connected interface
structure is being deleted, but eigrp was keeping a pointer to
the connected structure.  At the same time we were keeping the address
of the connected structure and this is all we need, so keep a copy
of that data and use that instead.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>