inspector: Always underline at least one char

Empty underlines are hard to make out. Since we get somewhat
unreliable section information from the CSS parser, we just
make sure that we always underline at least one character.

gtk/inspector/css-editor.c

@@ -337,6 +337,9 @@ show_parsing_error (GtkCssProvider        *provider,
   else
     tag_name = "error";
 
+  if (gtk_text_iter_equal (&css_error->start, &css_error->end))
+    gtk_text_iter_forward_char (&css_error->end);
+
   gtk_text_buffer_apply_tag_by_name (buffer, tag_name, &css_error->start, &css_error->end);
 
   ce->priv->errors = g_list_prepend (ce->priv->errors, css_error);