Handle non-PV $_ in @INC filters

Handle non-PV $_ in @INC filters

@INC filters (code refs returned by code refs in @INC) are given the
current line of code in $_ and can modify it.  The C code that invokes
the Perl filter is in pp_ctl.c:S_run_user_filter.  It was not taking
into account that $_ might not have a PV pointer when it is returned,
and so this could result in crashes or assertion failures.

This commit forces the scalar to be a string before returning it to
the lexer, unless it is undef.  If we force it to be a string when it
is undef, then existing tests start producing uninitialized warnings.

The logic is still faulty in places.  Subsequent commits will
address that.