In d1874b4339, we adjusted this check so that we insist on
using routerinfos for bridges. That's almost correct... but if we
have a bridge that is also a regular relay, then we should use
insist on its routerinfo when connecting to it as a bridge
(directly), and be willing to use its microdescriptor when
connecting to it elsewhere in our circuits.
This bug is a likely cause of some (all?) of the (exit_ei == NULL)
failures we've been seeing.
Fixes bug 25691; bugfix on 0.3.3.4-alpha
