close($undef) should not croak_no_modify

close($undef) should not croak_no_modify

Commit ac53db4c3f7e fixed bug #31767 (open $1 not dying), but put the
SvREADONLY check in the wrong spot, causing this bug:

$ perl -lwe 'no warnings "once"; close $x; close $+'
Name "main::x" used only once: possible typo at -e line 1.
Use of uninitialized value $x in ref-to-glob cast at -e line 1.
Modification of a read-only value attempted at -e line 1.

It shouldn’t be dying if I’m not trying to modifying it.