fdo#72788: sw: fix infinite loop in SwTxtAttr::RstTxtAttr

The increment was erroneously moved into the else branch in commit
c1c5feaff453cfbb23c43f2b423201b883ff048d.

Change-Id: If71cb58967e215548aed5679da86af20de17a0f1
(cherry picked from commit 8c59c30153b8a67d9455d70a66b4a42c35120f97)
Reviewed-on: https://gerrit.libreoffice.org/7414
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>