When deciding whether to discard interrupted lock/unlock context while
changing levels, maybe_reset_pick() checks whether xlock.box is being
carried. But it was doing so after the old level had been saved and
memory for non-carried container there had been freed.
That led to a couple of other issues. context.travelcc was using -1
for 'no cached value', but the fields of travelcc have type 'xchar' and
shouldn't be given negative values. 0 should be fine for 'no cache'.
Failed partial restore which occurred after old game's context had been
loaded would begin a new game with old game's stale context. Restoring
goes out of its way to avoid that for 'flags' but didn't for 'context'.
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