into_string() doesn't check `malloc` return value.

[KokaKiwi]

When doing a simple line like "toto".into_string(), the generated code doesn't have any malloc return value check (which is done with boxxed values).

Issue code here: https://gist.github.com/KokaKiwi/b33f7526185f823fc99a (My Rust version is in the Gist)