mapbox: don't try to emit 'plotly_relayout' after map.remove()

- see http://codepen.io/etpinard/pen/ZBabVW
  for explanation
- this removes the 'gd.emit is not a function' failures on
  'npm run citest-jasmine'

Author: etpinard

src/plots/mapbox/mapbox.js

@@ -126,6 +126,8 @@ proto.createMap = function(calcData, fullLayout, resolve, reject) {
 
     // keep track of pan / zoom in user layout and emit relayout event
     map.on('moveend', function(eventData) {
+        if(!self.map) return;
+
         var view = self.getView();
 
         opts._input.center = opts.center = view.center;
@@ -358,7 +360,10 @@ proto.updateLayers = function() {
 };
 
 proto.destroy = function() {
-    if(this.map) this.map.remove();
+    if(this.map) {
+        this.map.remove();
+        this.map = null;
+    }
     this.container.removeChild(this.div);
 };