Fix: GPv3: Draw Tool: Prevent trim_end_points from removing entire stroke

The `trim_end_points` in the draw tool was only supposed to be able to remove
end points of the stroke.
Make sure to clamp the value so that at least one point remains in the curve.

source/blender/editors/sculpt_paint/grease_pencil_paint.cc

@@ -1266,6 +1266,11 @@ static int trim_end_points(bke::greasepencil::Drawing &drawing,
     return 0;
   }
 
+  /* Don't remove the entire stroke. Leave at least one point. */
+  if (points.size() - num_points_to_remove < 1) {
+    num_points_to_remove = points.size() - 1;
+  }
+
   if (!on_back) {
     curves.resize(curves.points_num() - num_points_to_remove, curves.curves_num());
     curves.offsets_for_write().last() = curves.points_num();