Hello world samples are not working

[jgautier]

Describe the bug

fd_write for stdout does not appear to be working. I tried running the example here (https://wapm.io/package/chai2010/wasi-hello) using wapm and did not see any output. Then I ran the example here https://github.com/CraneStation/wasmtime/blob/master/docs/WASI-tutorial.md#web-assembly-text-example using wasmer and also did not see any output.

wasmer 0.7.0 | rustc 1.34.2 (6c2484dc3 2019-05-13) | x86_64

Steps to reproduce

1. wapm install chai2010/wasi-hello
2. wapm run wasi-hello

Expected behavior

see hello world printed to console

Actual behavior

nothing is printed

[MarkMcCaskey]

Thanks for the issue! This is actually a null-pointer deref issue. We don't deref null pointers from Wasm, using any memory address but 0 should work.

I'll look into what the standard says, but checking null has helped us catch bugs in the Wasm earlier in the past

[MarkMcCaskey]

It seems that null deref is fine in general, but not for Emscripten. I'll move the null-checks from runtime-core into emscripten -- thanks!

[MarkMcCaskey]

This now works on master, let me know if you have any other issues!