Fix assertion failure with $#a=\1

Fix assertion failure with $#a=\1

If the array has been freed and a reference is then assigned to
the arylen scalar and then get-magic is called on that scalar,
Perl_magic_getarylen misbehaves.  SvOK_off is not sufficient if
arbitrary values can be assigned by Perl code.  Globs, refs and
regexps (among others) need special handling, which sv_setsv
knows how to do.