BUG: Number of points exceed number of scalars in test

TestXYPlot inserted an extra point (dependent variable) for each
scalar (independent variable). XYPlotActor did not detect this and
valgrind reported "Invalid read of size 8" defects.

This patch removes the extra inserted point from the test and also
checks for this error condition in XYPlotActor.

Change-Id: Ic585534fdfbd665b708845b648fb30dee541475e
Merge topic 'InvalidRead_XYPlotActor' into master

3e3d3db2 BUG: Number of points exceed number of scalars in test