progress_cb: avoid buffer overflow

The progress bar output function would blindly use the terminal width
without bounds checking. When using a very wide terminal that caused a
buffer overflow and segfault.

We now limit the max bar with to 255 columns, and I simplified the code
to avoid an extra snprintf and buffer.

Bug: http://curl.haxx.se/bug/view.cgi?id=3435710
Reported by: Alexey Zakhlestin