SofQW3 segfault

[mducle]

Fixes an out-of-bounds index error in FractionalRebinning which causes a segmentation fault when SofQW3 is called with certain parameters.

To test:

Run the following script:

ws = CreateSampleWorkspace(binWidth = 0.75, XMin = 0, XMax = 300, XUnit = 'DeltaE')
ws = ScaleX(ws, -150, "Add")
LoadInstrument(ws, InstrumentName='MARI', RewriteSpectraMap = True)
ws_sqw = SofQW3(ws, '0,10,10', 'Direct', 150, EAxisBinning='60,0.75,90')

And check that no segfault occurs.

Fixes #22191.

Release Notes

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[martyngigg]

Is it correct that the next statement would now only fire if xend==x_start ans y_end==y_start?

[mducle]

Doh! I think I completely missed the point. I just looked at the loop in the general code (calcGeneralIntersection) and thought that it meant that when [xy]_start==[xy]_end meant there is only one bin width, and that I had misunderstood what [xy]_end meant. But actually the code before was right.

What [xy]_start == [xy]_end means is that the bin width in that direction is zero, and this was what was causing the segmentation fault.

So, actually we can fix the segfault without changing so much of the code. I'll push another commit to do this...

[martyngigg]

@mducle Are there any extra tests that are worth adding here?

[martyngigg]

The extra protection avoids the crash when encountering a bin with zero width. Good to go and is safe to include in the patch.