getLastFinishedBuild() fails with simultaneous builds

[maruel]

If you connect 2 slaves on a builder and call IBuilderStatus.getLastFinishedBuild() on the builder, it will fail to return the last completed build if both are building simultaneously.

This function should be changed to do a loop instead but a trivial loop won't work since IBuilderStatus.getBuild() may return None when it has pending requests, it should still be clamped when it hits the end of the build list in case there is no finished build yet.

This currently breaks console view with simultaneous builds.

I want to write a unit test first. :)