Getting the product's run environment via qbs API crashes if product is disabled

On master branch when testing the Xcode generator, I was able to trigger a crash when attempting to retrieve the run environment of a disabled product.

The crash occurs in artifactsMap() in src/lib/corelib/buildgraph/artifactsscriptvalue.cpp: the body of the function contains the statement 

return product->buildData->artifactsByFileTag();

which results in a null pointer dereference because product->buildData is null in this case.