In-place sort should not leave array read-only

In-place sort should not leave array read-only

$ ./perl -Ilib -e '@a=1..2; eval { @a=sort{die} @a }; warn "ok so far\n"; @a = 1'
ok so far
Modification of a read-only value attempted at -e line 1.

If something goes wrong inside the sort block and it dies, we still
need to make sure we turn off the read-only flag on that array.