fix exception in mouse event handler with empty CanvasMarkerLayer

`this._markers` is undefined in `_searchPoints`, called from event listeners.
Now we check markers existence in the beginning of handlers.

Note 1:
Could be fixed in some another place:
 e.g. check `_markers` in `_searchPoints`
 ...or initialize (empty) `_markers` on CanvasMarkerLayer init

Note 2:
`!this._map` removed from condition in `_onMouseMove:` 'cause
it's never should be true as we detach listeners on remove from map

src/plugin/leaflet.canvas-markers.js

@@ -278,6 +278,8 @@ function layerFactory(L) {
             return this;
         },
         _onClick: function (e) {
+            if (!this._markers) { return; }
+
             var self = this;
             var point = e.containerPoint;
 
@@ -298,7 +300,7 @@ function layerFactory(L) {
             }
         },
         _onMouseMove: function (e) {
-            if (!this._map || this._map.dragging.moving() || this._map._animatingZoom) { return; }
+            if (!this._markers || this._map.dragging.moving() || this._map._animatingZoom) { return; }
 
             var point = e.containerPoint;
             this._handleMouseHover(e, point);