Fuzz test a 32-bit build

[stoklund]

In the read_function_body() function, the code reading the local variable declarations looks like this:

        for _ in 0..local_count {
            let (count, ty) = self.reader.read_local_decl()?;
            locals_total += count as usize;
            if locals_total > MAX_WASM_FUNCTION_LOCALS {
                return Err(BinaryReaderError {
                               message: "local_count is out of bounds",
                               offset: self.reader.position - 1,
                           });
            }
            locals.push((count, ty));
        }

In a 32-bit build, the addition in locals_total += count as usize could overflow which causes a panic only in debug builds. In release builds it silently wraps.

A fuzz tester running on a 32-bit build would probably catch that.