Add a sanity check for a2c arguments

lib/a2c.js

@@ -137,6 +137,16 @@ module.exports = function a2c(x1, y1, x2, y2, fa, fs, rx, ry, φ) {
   var x1p =  cos_φ*(x1-x2)/2 + sin_φ*(y1-y2)/2;
   var y1p = -sin_φ*(x1-x2)/2 + cos_φ*(y1-y2)/2;
 
+  if (x1p === 0 && y1p === 0) {
+    // we're asked to draw line to itself
+    return [];
+  }
+
+  if (rx === 0 || ry === 0) {
+    // one of the radii is zero
+    return [];
+  }
+
   var radii = correct_radii(x1p, y1p, rx, ry);
   rx = radii[0];
   ry = radii[1];
@@ -152,7 +162,7 @@ module.exports = function a2c(x1, y1, x2, y2, fa, fs, rx, ry, φ) {
   // Split an arc to multiple segments, so each segment
   // will be less than τ/4 (= 90°)
   //
-  var segments = Math.ceil(Math.abs(Δθ) / (TAU / 4));
+  var segments = Math.max(Math.ceil(Math.abs(Δθ) / (TAU / 4)), 1);
   Δθ /= segments;
 
   for (var i = 0; i < segments; i++) {

test/api.js

@@ -425,5 +425,19 @@ describe('API', function () {
         'M100 100'
       );
     });
+
+    it('radii are zero', function () {
+      // both rx and ry are zero
+      assert.equal(
+        svgpath('M100 100A0 0 0 0 1 110 110').unarc().round().toString(),
+        'M100 100'
+      );
+
+      // rx is zero
+      assert.equal(
+        svgpath('M100 100A0 100 0 0 1 110 110').unarc().round().toString(),
+        'M100 100'
+      );
+    });
   });
 });