Fix binary search on lists of 0 or 1 element.

The implementation we added has a tendency to crash with lists of 0 or
one element.  That can happen if we get a consensus vote, v2
consensus, consensus, or geoip file with 0 or 1 element.  There's a
DOS opportunity there that authorities could exploit against one
another, and which an evil v2 authority could exploit against anything
downloading v2 directory information..

This fix is minimalistic: It just adds a special-case for 0- and
1-element lists.  For 0.2.4 (the current alpha series) we'll want a
better patch.

This is bug 7191; it's a fix on 0.2.0.10-alpha.