Add a few asserts. NFC.

src/mesh.cpp

@@ -289,8 +289,8 @@ void SMesh::MakeFromDifferenceOf(SMesh *a, SMesh *b) {
 }
 
 void SMesh::MakeFromCopyOf(SMesh *a) {
-    int i;
-    for(i = 0; i < a->l.n; i++) {
+    ssassert(this != a, "Can't make from copy of self");
+    for(int i = 0; i < a->l.n; i++) {
         AddTriangle(&(a->l.elem[i]));
     }
 }

src/srf/surface.cpp

@@ -829,6 +829,7 @@ void SShell::MakeFromRevolutionOf(SBezierLoopSet *sbls, Vector pt, Vector axis,
 }
 
 void SShell::MakeFromCopyOf(SShell *a) {
+    ssassert(this != a, "Can't make from copy of self");
     MakeFromTransformationOf(a,
         Vector::From(0, 0, 0), Quaternion::IDENTITY, 1.0);
 }