Somewhat "safer" _getSubdomain method - now it works even when (tilePoint.x + tilePoint.y) is negative.

Note that (tilePoint.x + tilePoint.y) might be negative in some cases (custom CRS with tiled WMS).
I discovered that tilePoint.y is often negative when using custom CRS (epsg:2393), tiled WMS layer and proj4leaflet together. 
This small fix prevents possible negative subdomain index generation.