Narrow fillet_corner_p3 null-curve guard and check curve count first

The shim no longer refuses a corner edge without a 3D curve: upstream
CreateCornerBlend returns SM_ERR below four curves and otherwise
SM_ERR_NULL_POINTER, without crashing. create_corner_patch now checks the
count before dereferencing curves, matching that order.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011gJAwcLRBPQXYLmvXd3pWf

crates/solidcore/src/blend/fillet_corner/p3.rs

@@ -1479,6 +1479,13 @@ impl FilletCorner {
             let _ = total_edges;
 
             // build the corner patch from the ordered array boundary information
+            // CreateCornerBlend checks `cr3DCurves.GetSize() < 4 -> SM_ERR` before it
+            // touches any curve (a null one is SM_ERR_NULL_POINTER afterwards).
+            if ordered_3d.len() < 4 {
+                return Err(SolidError::sm_err(
+                    "nurbs::b_spline_surface_n_lib::p2::BSplineSurface::create_corner_blend",
+                ));
+            }
             let mut curves_3d = Vec::with_capacity(ordered_3d.len());
             for c in &ordered_3d {
                 let c = c

harness/cpp/shim/blend_fillet_corner_p3_static.inc

@@ -587,8 +587,8 @@ double p3_check_default_patch(SmFilletCorner* c, const std::vector<SmFilletEdge*
             }
         }
         SmCurve* curve = fe == ov.edge ? ov.curve : fe->GetCurve();
-        if (curve == NULL)
-            return P3C_REFUSE; // CreateCornerBlend reads every boundary curve
+        // A NULL curve is not refused: CreateCornerBlend returns SM_ERR below four curves,
+        // else sm_CreateNonRationalCurve casts null-safely and returns SM_ERR_NULL_POINTER.
         uv.Add(ps);
         c3d.Add(curve);
         orients.Add(SM_OT_SAME);