I noticed that `TableOp::insert` had assertions that `num_params` and
`table_size` were greater than 0, but no assert for `num_globals`. These
asserts couldn't be hit because the `*_RANGE` constants were all set to
a minimum of 1.
But the only reason I can see to prohibit 0-sized tables, locals, or
globals, was because indexes into those spaces were generated with the
`%` operator. Allowing 0-sized spaces requires not generating the
corresponding instructions at all when there are no valid indexes.
So I pushed the final selection of which table/local/global to access
earlier, to the moment when we're picking which TableOps to run. Then,
instead of generating a random u8 or u32 and taking the remainder to get
it into the right range, I can just ask `arbitrary` to generate a number
in the right range to begin with.
So this now explores some size-0 corners that it didn't before, and it
doesn't require reasoning about whether remainder can divide by zero.
Also I think it uses fewer bits of the `Unstructured` input to produce
the same cases, and I hope that lets libFuzzer more quickly find bits it
can mutate to get to novel coverage paths.
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