WeakPointer: Check for exclusive write access in LengthWPObj

[markuspf]

• As per Reimer's suggestion check for exclusive write access in
  LengthWPObj.
• If we have write access, we discard objects at the end of the
  list, otherwise we just return the stored length.

[markuspf]

@rbehrends is this ok? I realised the patch might be overly complicated, one could just put the following at the beginning of the function.

if(!CheckExclusiveWriteAccess(wp) {
  return STORED_LEN(wp);
}

[rbehrends]

Both should work, as far as I can tell.