Fix uvsculpt null pointer reference in paint.c

ASAN reported null pointer access when going into edit mode on the
default cube.

Check in uvculpt has been initialized before trying to use it.

source/blender/blenkernel/intern/paint.c

@@ -406,7 +406,7 @@ Paint *BKE_paint_get_active(Scene *sce, ViewLayer *view_layer)
         case OB_MODE_WEIGHT_GPENCIL:
           return &ts->gp_weightpaint->paint;
         case OB_MODE_EDIT:
-          return &ts->uvsculpt->paint;
+          return ts->uvsculpt ? &ts->uvsculpt->paint : NULL;
         default:
           break;
       }