When making sure digest256 matches in ei, look at sd, not ri.

The routerinfo we pass to routerinfo_incompatible_with_extrainfo is
the latest routerinfo for the relay.  The signed_descriptor_t, on
the other hand, is the signed_descriptor_t that corresponds to the
extrainfo.  That means we should be checking the digest256 match
with that signed_descriptor_t, not with the routerinfo.

Fixes bug 17150 (and 19017); bugfix on 0.2.7.2-alpha.