textview: handle NULL cursors gracefully
textview: handle NULL cursors gracefully

Under some cases, we can get NULL back for the cursor. Simply use
g_clear_object() to handle that case gracefully.

chergert/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,alexlarsson/gtk,davidgumberg/gtk,jigpu/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,chergert/gtk,davidgumberg/gtk,jadahl/gtk,alexlarsson/gtk,chergert/gtk,alexlarsson/gtk,jadahl/gtk,chergert/gtk,chergert/gtk,grubersjoe/adwaita,jadahl/gtk,chergert/gtk,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,jigpu/gtk,chergert/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,davidgumberg/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,chergert/gtk