poly2d/p2d_attrib.c (p2d_is_cw): check that the angle isn't NaN

Been had by this a little too often ...

poly2d/p2d_attrib.c

@@ -72,6 +72,7 @@ bool p2d_is_cw(const struct p2d *p)
 		v = v->next;
 	}
 	while (v != p->v);
+	assert(!isnan(a));	/* require -std=c99 or -std=gnu99 */
 	return a < 0;
 }