ensure compile error doesn't lose preview position
ensure compile error doesn't lose preview position

When the preview pane is displaying a compile error, calls to
previewgui_save_position would previously reset the saved position to
0/0, and thus the position in the preview would be lost when the error
was cleared. This change prevents calls to previewgui_save_position from
having any effect when an error is being displayed.

alexandervdm/gummi,alexandervdm/gummi,alexandervdm/gummi