add asserts when scanfilling or triangulating with zero length normal.

source/blender/blenlib/intern/scanfill.c

@@ -869,6 +869,8 @@ int BLI_scanfill_calc_ex(ScanFillContext *sf_ctx, const int flag, const float no
 	int totfaces = 0; /* total faces added */
 	float mat_2d[3][3];
 
+	BLI_assert(!nor_proj || len_squared_v3(nor_proj) > FLT_EPSILON);
+
 	/* reset variables */
 	eve = sf_ctx->fillvertbase.first;
 	a = 0;

source/blender/bmesh/intern/bmesh_polygon.c

@@ -831,6 +831,8 @@ static BMLoop *poly_find_ear(BMFace *f, float (*projectverts)[2], const bool use
 	const float cos_threshold = 0.9f;
 	const float bias = 1.0f + 1e-6f;
 
+	BLI_assert(len_squared_v3(f->no) > FLT_EPSILON);
+
 	if (f->len == 4) {
 		BMLoop *larr[4];
 		int i = 0, i4;