Fix read-only flag checks in lvalue sub exit

Fix read-only flag checks in lvalue sub exit

See the previous commit for the explanation.  This fixes this
discrepancy:

$ ./miniperl -Ilib -e '+sub:lvalue{my $x = 3; Internals::SvREADONLY $x, 1; $x }->() = 3'
Can't return a readonly value from lvalue subroutine at -e line 1.
$ ./miniperl -Ilib -e '+sub:lvalue{my $x = *foo; Internals::SvREADONLY $x, 1; $x }->() = 3'
Modification of a read-only value attempted at -e line 1.

It was not just a flag check that this commit fixed, but also a bogus
SvREADONLY(TOPs) where TOPs may not even be the scalar we are dying
for, giving ‘a temporary’ for some read-only values.  That mistake was
my own, made in commit d25b0d7b8.