[RT #78266] Don't leak memory when accessing named captures that didn't match

[RT #78266] Don't leak memory when accessing named captures that didn't match

Since 5.10 (probably 44a2ac759e) named captures have been leaking
memory when they're used, don't actually match, but are later
accessed. E.g.:

    $ perl -wle 'for (1..10_000_000) { if ("foo" =~ /(foo|(?<capture>bar))?/) { my $capture = $+{capture} } } system "ps -o rss $$"'
      RSS
    238524

Here we match the "foo" branch of our regex, but since we've used a
name capture we'll end up running the code in
Perl_reg_named_buff_fetch, which allocates a newSVsv(&PL_sv_undef) but
never uses it unless it's trying to return an array.

Just change that code not to allocate scalars we don't plan to
return. With this fix we don't leak any memory since there's nothing
to leak anymore.

    $ ./perl -Ilib -wle 'for (1..10_000_000) { if ("foo" =~ /(foo|(?<capture>bar))?/) { my $capture = $+{capture} } } system "ps -o rss $$"'
      RSS
     3528

This reverts commit b28f4af8cf94eb18c0cfde71e9625081912499a8 ("Fix
allocating something in the first place is a better solution than
allocating it, not using it, and then freeing it.