Fixes rusti crashes.

Don't pass the binary name to the LLVMRustExecuteJIT closure; doesn't
seem to be needed, anyhow.
auto merge of #6397 : cantsin/rust/rusti-fixes, r=catamorphism

Fixes #6378

Don't pass the binary name to the LLVMRustExecuteJIT closure, otherwise it will leak memory; the binary name doesn't seem to be needed, anyhow.