Fix crash when require is called from root VM stack

[vrn-sn]

Copied from #1785:

If require is called from the root interpreter stack (e.g. using C API) then lua_getinfo call will not succeed, leaving garbage in lua_Debug ar struct.
Accessing later ar.source as null-terminated string is unsafe and can cause a crash.

This PR adds a check to ensure that lua_getinfo call is successful.