Prevent to redraw a marker when setting invalid latlng from edit form

cf #799

umap/static/umap/js/umap.core.js

@@ -546,3 +546,7 @@ L.U.Orderable = L.Evented.extend({
     }
 
 });
+
+L.LatLng.prototype.isValid = function () {
+    return !isNaN(this.lat) && !isNaN(this.lng);
+}

umap/static/umap/js/umap.features.js

@@ -585,6 +585,7 @@ L.U.Marker = L.Marker.extend({
         ];
         var builder = new L.U.FormBuilder(this, coordinatesOptions, {
             callback: function () {
+                if (!this._latlng.isValid()) return;
                 this._redraw();
                 this.bringToCenter();
             },