Fix rare use of undefined variable

[starboerg]

Variable x2 was used undefined in case of polygons without y-extend. Defaulting x2 to zero works around this issue.

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[magjac]

@starboerg Thanks for the contribution and apologies for the delay. Can you provide an example that trigger this? Or even better, add a test case?

[starboerg]

@magjac I did not dive too deep into the code, just stumbled upon this issue. I use a sequence of graphs and transition them into each other:
graphviz.transition(function () { return d3.transition("main").ease(d3.easeLinear).delay(0).duration(500); });
Likely a horizontal line element in the graphs triggers this, but I did not keep any example.