Fix overflow errors and allow small terminal sizes - #473

* In #259 users request the ability to have ncmpcpp remain open when the
  terminal has a small size. Previously ncmpcpp would exit when there
  were less than 30 columns or 5 rows.
* After removing this constraint, ncmpcpp would freeze when terminal
  sizes went below those thresholds.
* Upon debugging, it became clear that ncmpcpp wasn't frozen, it was
  stuck in a very long for loop because it thought the display was
  unreasonably large (resulting from integer overflow).
* Preventing that overflow allows ncmpcpp to handle small sizes
  gracefully. No other changes to the rendering code were necessary.

Fixes #259