load_arr: don't assert on early-ending strings
load_arr: don't assert on early-ending strings

This can happen legitimately on first load if the string doesn't end
with an EOL (for 2d) or EOP (for 3d). I think it can also happen later
on if we load something that fits exactly in the beginning of an
existing box, like the following:

+----------------+
| loading into   |
| this part      |
+................+
| existing box   |
| continues here |
+----------------+

Deewiant/mushspace,Deewiant/mushspace,Deewiant/mushspace