Fixes repaint problem with $pager_index_lines #159

Fixes repaint problem with $pager_index_lines #159

Commit 6b4c4ca changed the behaviour of mutt_pager to build the
pager index even if pager_index_lines was set to 0 to (fixing #143).

The code redrawing the pager status line was relying on the pager index
being NULL to determine if a redraw should take place. Now
index_window->rows is used as an additional safeguard.

Also added a check if pager index is set to the terminal status line
update code as it was unguarded and old default behaviour might be
introduced later.

Fixes #159