Don’t taint undef in reset
reset was tainting undef if the internal SV type happened to be SVt_PV
or higher. This has got to have been a mistake. Tainting undef or
what is known to be an empty string does not make sense, even in a
tainted expression. Tainting it based on the internal type does not
make sense either, and results in inconsistencies in behaviour (taint
it if it *was* a string, even though it isn’t now, but not if it was a
number, unless that number was tied, or had pos() set, etc.).
This tainting has been here since perl 3.0 (commit
a687059cb), which I
think was when tainting was introduced.
Applying the tainting based on the internal type has happened since
79072805bf6 (perl 5.0 alpha 2), which introduced different internal
SV types.