BMesh: avoid assert dissolving degenerate faces

For dissolving 2-sided faces, theres no need to check they are valid before removal.

source/blender/bmesh/intern/bmesh_core.c

@@ -872,7 +872,12 @@ void BM_face_kill(BMesh *bm, BMFace *f)
 	BMLoopList *ls, *ls_next;
 #endif
 
-	BM_CHECK_ELEMENT(f);
+#ifdef NDEBUG
+	/* check length since we may be removing degenerate faces */
+	if (f->len >= 3) {
+		BM_CHECK_ELEMENT(f);
+	}
+#endif
 
 #ifdef USE_BMESH_HOLES
 	for (ls = f->loops.first; ls; ls = ls_next)