Fix crash on submap with plant without seed

The plant furniture expects there to be a seed item on that square.
This is also expected by the iexamine function.
If the seed is gone (for whatever reason), change the plant into something else.
Fix access to current overmap before it exists.

When loading an old save, the map system must be converted to the new system.
That code expects a current overmap to get coordinates, but the game is not loaded an therefor game::cur_om is not set.
Don't display scrollingcombattext outside of the terrain window.
Prevent scrolling combat text from npcs