[setup topic/libsolv] Crash after incomplete download

[Ken Brown]

If there is a download failure and the user clicks Yes in response to 
"Download Incomplete.  Try again?", then setup will crash.  The crash 
occurs at PickView.cc:447 because i->source() is NULL.

I haven't yet analyzed this in further detail, but the crux of the issue 
seems to be that we call do_ini_thread a second time without having 
cleaned out the package database and the libsolv pool.

This is probably a symptom of a more general problem, which is that we 
haven't thought carefully (or at least I haven't) about what happens 
when we visit certain pages for a second time, after the libsolv pool 
has been created.

Before I work further on this, I have a UI question.  Is it really 
reasonable that we go back to the mirror selection page after "Download 
incomplete"?  I understand the rationale, which is that the user might 
want to try a different mirror after a download failure.  But I 
personally have always found this annoying.  I would rather just retry 
the download, which is what the message ("Download Incomplete.  Try 
again?") suggests is going to happen.

Ken