force-build can cause traceback on Builder page

[warner]

I did force-build via the web page, and the build took a little while to get going. (actually, it was waiting for a SlaveLock?.. that's probably why there was no current step). When the page reloaded and it tried to tell me about the build that was currently running, I got the following exception:

/usr/local/stow/buildbot-0.7.6/lib/python2.3/site-packages/buildbot/status/web/builder.py, line 60 in body
60        data += " <li>" + self.build_line(build, req) + "</li>
"
/usr/local/stow/buildbot-0.7.6/lib/python2.3/site-packages/buildbot/status/web/builder.py, line 36 in build_line
36    data += "[%s]" % build.getCurrentStep().getName()
exceptions.AttributeError: 'NoneType' object has no attribute 'getName'

[warner]

Oops, just like the problem in #96. I've applied a fix in [1c3e315e4c827f38c987993f06fdb7ca562b5d36] that ought to take care of it.