recursive_tokenize('foo => {') segfaults

[jhi]

Found by accident while minimizing another crash case.

[Code-Hex]

I'm thinking that the cause of this issue is get_used_modules. This issue also causes the same crash when get_used_modules is executed. And since recursive_tokenize calls get_used_modules internally, this kind of crash is occurring.

In other words, we need to fix get_used_modules to solve this issue.