Add guard around zero width or height

[Bengreen]

in my usage of this via leaflet.heat I have a lifecycle where the canvas can be 0 width or 0 height which is causing getImageData to throw an error. I placed a guard around the relevant section so this case is covered. If you can accept this request I will contact the leaflet.heat upstream and ask for them to propagate the update to their release.

[Bengreen]

Any feedback if this can be included ?

[Dumbaz]

This has been open for a long time, was it just an oversight?

[corrigancd]

Hello, great plugin. Wondering if this is something that can be merged?

[victorperezpiqueras]

Same issue here, is it going to be merged anytime soon?