Fix a bug introduced in #4420: it always walked back until it reaches blocks in the this chain.
This is currently not exposed, as we always call CChain::GetLocator with blocks in the main chain, but in #4468 it gets used for more.
Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/p4470_f4b00beae52f22fc6958855a85e1012387c3c33e/ for binaries and test log. This test script verifies pulls every time they are updated. It, however, dies sometimes and fails to test properly. If you are waiting on a test, please check timestamps to verify that the test.log is moving at http://jenkins.bluematt.me/pull-tester/current/ Contact BlueMatt on freenode if something looks broken.
Untested ACK