Sculpt: Fix crash when using grab brush

Was wrong paint structure deducted from context.

Test plan:

With default cube, go to sculpt mode, switch to grab brush
and try to sculpt.

source/blender/blenkernel/intern/paint.c

@@ -214,8 +214,7 @@ Paint *BKE_paint_get_active_from_context(const bContext *C)
 			}
 		}
 		else {
-			/* default to image paint */
-			return &ts->imapaint.paint;
+			return BKE_paint_get_active(sce, view_layer);
 		}
 	}