VIDEO: Fix memory leak in TrueMotion1 header buffer

Cleanup _buf before returning, even if we don't actually decode
WINTERMUTE: Don't clean up _scripts mid-iteration

Scripts executing from ScEngine::tick() can call
ScEngine::unbreakableTick() via scCallMethod("SaveGame") and
applyEvent("BeforeSave").

This recursive execution could cause finished scripts being removed
from _scripts while ScEngine::tick() is still iterating over that array.
WINTERMUTE: Disambiguate empty and NULL strings when saving

The string stored is now strlen(s)+1, with length 0 indicating NULL.

Increment savegame version for this new format.

Old savegames are fixed by assuming VAL_STRING should never be NULL.