cui: fix Impress hyperlink insert when the form listbox is hidden

In preparation of forward-porting the distro/collabora/cp-6.0-only
commit 418adc09 (lo: disable under-used
hyperlink fields that are buggy online., 2019-05-22), which relaxes the
invariant that mpLbForm never returns LISTBOX_ENTRY_NOTFOUND.

A signed integer overflow is a problem in itself, but in practice the
SID_HYPERLINK_SETLINK handler in sd::DrawViewShell::FuTemporary() didn't
insert a field, since the insert mode was a large negative value instead
of HLINK_FIELD.

Change-Id: I238d9f662a74f320d05ea6a6cc97319d2d5b3d5a
Reviewed-on: https://gerrit.libreoffice.org/73034Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

cui/source/dialogs/hltpbase.cxx

@@ -426,7 +426,13 @@ void SvxHyperlinkTabPageBase::GetDataFromCommonFields( OUString& aStrName,
     aStrIntName = mpEdText->GetText();
     aStrName    = mpEdIndication->GetText();
     aStrFrame   = mpCbbFrame->GetText();
-    eMode       = static_cast<SvxLinkInsertMode>(mpLbForm->GetSelectedEntryPos()+1);
+
+    sal_Int32 nPos = mpLbForm->GetSelectedEntryPos();
+    if (nPos == LISTBOX_ENTRY_NOTFOUND)
+        // This happens when FillStandardDlgFields() hides mpLbForm.
+        nPos = 0;
+    eMode = static_cast<SvxLinkInsertMode>(nPos + 1);
+
     // Ask dialog whether the current doc is a HTML-doc
     if (static_cast<SvxHpLinkDlg*>(mpDialog.get())->IsHTMLDoc())
         eMode = static_cast<SvxLinkInsertMode>( sal_uInt16(eMode) | HLINK_HTMLMODE );