Throw error on call to L.Polygon getCenter before map add (#4820)

* Throw error on call to L.Polygon getCenter before map add

References #4740

* add polyline handling, tests, and docstring

spec/suites/layer/vector/PolygonSpec.js

@@ -162,6 +162,16 @@ describe('Polygon', function () {
 			expect(layer.getCenter()).to.be.nearLatLng(L.latLng([0, 0]));
 		});
 
+		it('throws error if not yet added to map', function () {
+			expect(function () {
+				var latlngs = [
+					[[0, 0], [10, 0], [10, 10], [0, 10]]
+				];
+				var layer = new L.Polygon(latlngs);
+				var center = layer.getCenter();
+			}).to.throwException('Must add layer to map before using getCenter()');
+		});
+
 	});
 
 	describe("#_defaultShape", function () {

spec/suites/layer/vector/PolylineSpec.js

@@ -135,6 +135,13 @@ describe('Polyline', function () {
 			expect(polyline.getCenter()).to.be.nearLatLng(L.latLng([0, 0]), 1e-2);
 		});
 
+		it('throws error if not yet added to map', function () {
+			expect(function () {
+				var polyline = new L.Polyline([[0, 0], [0, 0.090]]);
+				var center = polyline.getCenter();
+			}).to.throwException('Must add layer to map before using getCenter()');
+		});
+
 	});
 
 	describe('#_flat', function () {

src/layer/vector/Polygon.js

@@ -55,6 +55,11 @@ L.Polygon = L.Polyline.extend({
 	},
 
 	getCenter: function () {
+		// throws error when not yet added to map as this center calculation requires projected coordinates
+		if (!this._map) {
+			throw new Error('Must add layer to map before using getCenter()');
+		}
+
 		var i, j, p1, p2, f, area, x, y, center,
 		    points = this._rings[0],
 		    len = points.length;

src/layer/vector/Polyline.js

@@ -105,6 +105,11 @@ L.Polyline = L.Path.extend({
 	// @method getCenter(): LatLng
 	// Returns the center ([centroid](http://en.wikipedia.org/wiki/Centroid)) of the polyline.
 	getCenter: function () {
+		// throws error when not yet added to map as this center calculation requires projected coordinates
+		if (!this._map) {
+			throw new Error('Must add layer to map before using getCenter()');
+		}
+
 		var i, halfDist, segDist, dist, p1, p2, ratio,
 		    points = this._rings[0],
 		    len = points.length;