[perl #128182] Fix crash with require $nonstring

If something other than a plain string (e.g. a reference or typeglob)
whose stringified form contains a null character is passed to require()
or do(), it crashes, as of v5.19.3-130-gc8028aa, because the code in
question that handles the error tries to read fields of the scalar
that are only valid if it is a string internally.