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WMSTileFuser not responding for some extents

It turned out that an old gridsubset extent was written into the
xml-file for the layer. GridCoverage.getIntersection() doesn't handle
null intersection. It reports tiles outside of the requested  bounds.
This resulted in an exception in WMSTileFuser.renderCanvas that wasn't
handled --- e.printStackTrace().

I've fixed that the exceptions are logged and that extents outside the
tiles results in a blank canvas.