Fix address overflow bug in wasm2c (#1401)

This only occurs when the immediate offset is small (`int` sized). The
stack offset is `u32` and the immediate is an `int`, so the usual
arithmetic conversions converts the result to a `u32`, which wraps the
address before checking for overflow.

There are already spec tests for overflow, but these use an offset of
`4294967295`, which is `long` (at least on LP64 systems). This means
that the sum's type is `u32 + long` which is `long`. This is why the
tests pass. I've added additional tests for these cases here:
https://github.com/WebAssembly/spec/pull/1188

This fixes issue #1400.

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