Speed up SwAttrIter::GetNextAttr()

The inner loop which iterates over the characters of
m_pTxtNode->GetTxt() is already bounded to the length of the
string, so there's no need to pay the price of checking its length
for each array position

Change-Id: I7674ea2b46db75fea30dd016b96ec932068fd73b
Reviewed-on: https://gerrit.libreoffice.org/11784
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>