when unCOWing a string, set SvCUR to 0

When a COW string is unCOWed, as well as setting SvPVX to NULL and SvLEN
to 0, set SvCUR to 0 too.

This is to avoid a later SvGROW on the same using the old SvCUR() value
to calculate a roundup to the buffer size.

Consider the following code:

    use Devel::Peek;
    for (1..3) {
        my $t;
        my $s = 'x' x 100;
        $t = $s;
        Dump $s;
    }

Looking at the LEN line of the Dump output, we got on 5.20.0:

  LEN = 102
  LEN = 135
  LEN = 135

and after this commit,

  LEN = 102
  LEN = 102
  LEN = 102

As well as wasting space, this extra LEN was then triggering the 'skip COW
if LEN >> CUR' mechanism, causing extra copies. See:

    [perl #121977] COWification seems expensive in PADMY variables