Zero division fix (on leaflet.0.8-dev branch)

[olzraiti]

If the map was mounted to a node that's not in the DOM (which is not so rare for example in React environment), an error was thrown because pattern.getComputedTextLength() resulted in 0, which caused a division by zero. I fixed the bug by checking if the map is rendered in the body.

You can reproduce the bug like this:

document.createElement("div").appendChild(mapContainerNode);
map.invalidateSize();

[olzraiti]

I updated the code to just fix with 1 instead of 0. Much more fool proof and works on every case.

[olzraiti]

Closed by #79