Related: tdf#116951 use same pattern for gtk2

Change-Id: Ic6f1a018b4d0016af5a19052ef46ea87c7923acb
Reviewed-on: https://gerrit.libreoffice.org/53416Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/unx/gtk/gtksalframe.cxx

@@ -3559,8 +3559,14 @@ void GtkSalFrame::IMHandler::signalIMPreeditChanged( GtkIMContext*, gpointer im_
         ExtTextInputAttr sal_attr = ExtTextInputAttr::NONE;
 
         pango_attr_iterator_range (iter, &start, &end);
-        if (end == G_MAXINT)
-            end = pText ? g_utf8_strlen(pText, -1) : 0;
+        if (start == G_MAXINT || end == G_MAXINT)
+        {
+            auto len = pText ? g_utf8_strlen(pText, -1) : 0;
+            if (end == G_MAXINT)
+                end = len;
+            if (start == G_MAXINT)
+                start = len;
+        }
         if (end == start)
             continue;