msg/msg_type: entity_addr_t: fix legacy decode

msg/msg_type: entity_addr_t: fix legacy decode

If we decode a zeroed sockaddr, we should end up with a TYPE_NONE
entity_addr_t, not v1::/0.

This was obscured by unit test TestAddrvecEncodeAddrDecode3, which
took an addrvec with all v2 addrs, decoded to an addr variable that
previously had v1:1.2.3.4:/0, and asserted the result was not v1::/0.
The test passed before because the set_sockaddr() failed on AF_UNSPEC
and the addr kept v1:1.2.3.4, but with the previous commit it failed
because it equaled v1::/0.  In reality, addr should get - (addr TYPE_NONE).

The TestEmptyAddrvecEncodeAddrDecode test case is similarly adjusted.

Signed-off-by: Sage Weil <sage@redhat.com>