* Panic on resetting image slots back to anonymous memory
This commit updates `Drop for MemoryImageSlot` to panic instead of
ignoring errors when resetting memory back to a clean slate. On reading
some of this code again for a different change I realized that if an
error happens in `reset_with_anon_memory` it would be possible,
depending on where another error happened, to leak memory from one image
to another.
For example if `clear_and_remain_ready` failed its `madvise` (for
whatever reason) and didn't actually reset any memory, then if `Drop for
MemoryImageSlot` also hit an error trying to remap memory (for whatever
reason), then nothing about memory has changed and when the
`MemoryImageSlot` is recreated it'll think that it's 0-length when
actually it's a bit larger and may leak data.
I don't think this is a serious problem since we don't know any
situation under which the `madvise` would fail and/or the resetting with
anonymous memory, but given that these aren't expected to fail I figure
it's best to be a bit more defensive here and/or loud about failures.
* Update a comment
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