Avoid trying to evaluate arcs/circles/ellipses with 0 radius

git-svn-id: http://svn.clifford.at/openscad/trunk@381 b57f626f-c46c-0410-a088-ec61d464b74c

src/primitives.cc

@@ -194,6 +194,7 @@ void register_builtin_primitives()
 */
 int get_fragments_from_r(double r, double fn, double fs, double fa)
 {
+	if (r < GRID_FINE) return 0;
 	if (fn > 0.0)
 		return (int)fn;
 	return (int)ceil(fmax(fmin(360.0 / fa, r*M_PI / fs), 5));