Confusing log message when circuit can't be extended
add a changes file for bug 12718
Warn and drop the circuit if we receive an inbound 'relay early' cell

Those used to be normal to receive on hidden service circuits due to bug
1038, but the buggy Tor versions are long gone from the network so we
can afford to resume watching for them. Resolves the rest of bug 1038;
bugfix on 0.2.1.19.
Merge branch 'maint-0.2.4' into maint-0.2.5