Fix missing seed, scrolling combat text

Fixes #8100

Something else removed the seed of a plant. The game expects there to be a seed item and fails to recognize that it's gone.

Fix: replace the plant furniture with something else (I used hay here), to prevent other code (iexamine) from failing. Without the seed there is no way to know which type of plant there was. One could also use other furniture or no furniture at all, but hay gives at least a mark where a plant once was.

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There is an additional fix for converting old saves: `mapbuffer::save` would access `game::cur_om`, but that pointer is not set when the map loads (`MAPBUFFER.load` is called first in main_menu.cpp, than `game::load` and this sets `cur_om`). This results in a crash, which is now fixed.

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Also prevented the display of scrolling combat text that originates outside of the main terrain window. The save from the issue above has a nice example of that: there is a npc and a zombie dog to the northeast, outside of the main view. The zombie dog systematically destroys the armor of that npc and the messages about that appear near the player character.