Accept a valid path instead of an arbitrary string

[cmb69]

Since we're passing this parameter to a C function accepting a char*
without any further checking, we should reject strings with NUL bytes
in the first place.

[nikic]

Won't this give a somewhat odd error message though, because it refers to paths? (Or is this actually a path?)

[cmb69]

You're right (no it's not a path); haven't considered the error message. Guess we should have a strlen() check with appropriate error (message) instead – is there something readily available?

[cmb69]

Superseeded by #4553.