Slow to disassemble some crafted wasm files

wasm-dis may sometimes spend too much time to disassemble certain crafted wasm files.
wasm-dis-slow.zip

It turns out that inside WasmBinaryBuilder::readFunctions(), at https://github.com/WebAssembly/binaryen/blob/master/src/wasm/wasm-binary.cpp#L1073-L1080, the value num may get extremely large (I haven't figured out why yet), and subsequent operations require too much time to finish.

从提供的 wasm-dis-slow.zip 开始，检查 src/wasm/wasm-binary.cpp 中大约第 1073–1080 行的 WasmBinaryBuilder::readFunctions()，重点关注值 num 的获取和使用方式。复现缓慢的反汇编，并确定 num 为什么会变得极大。当构造的文件不再导致 wasm-dis 花费过多时间时，即表示完成。