Fix showing full percentage progress before metadata download start

[jan-kolarik]

There are some uninteresting progress events with zero values coming at the beginning of the metadata download which causes the percent widget to show 100% before the actual downloading starts.

As there is already implemented showing the ???%, I assume this was the original intention, so I fixed the incorrectly initialized members in the ProgressBar class.

This is a follow-up of #130

[jrohel]

Thanks. I know, it's a little PR. But can you please split it into two commits?

• The first commit then fixes the progress_bar implementation of the "progress_bar.hpp" and "progress_bar/cpp" files.
• The second commit will modify the "context.cpp" file of the dnf5 application.