Leaflet 1.3.0 compatibility

[Matthias Mohr]

It seems with Leaflet 1.3.0 this plugin is broken. It works fine until 1.2.0.

Chrome reports:

leaflet.js:5 Uncaught TypeError: Cannot read property 'tagName' of undefined
    at e._initImage (leaflet.js:5)
leaflet.js:5 Uncaught TypeError: Cannot set property '_leaflet_pos' of undefined
    at Object.Lt [as setPosition] (leaflet.js:5)

Can be reproduced in the demo by using Leaflet 1.3.0 instead of 1.0.3.