Prevent over-extension of vector length

[henrylay97]

Description

@RachelCoackley discovered this problem using the SBND CI. We were seeing non-reproducibility in the CAFs. @jzennamo traced this down to this tool using valgrind to find an invalid read. I have then investigated why it was happening.

Description copied from slack:

• The if statements here on lines 33 & 36 are not mutually exclusive and yet they both add a default value to the vector dtheta.
• If both evaluate to true then the vector grows by 2 entries during this iteration of the loop. This makes it longer than other vectors such as cumLenFwd and cumLenBwd which are meant to be of the same length.
• It is the dtheta vector's size (after passing to other functions) that determines the domain of the loop on line 224 (by setting beg & end at lines 211 & 212.
• This allows invalid reads of cumLen on lines 232 or 236.
• The simplest fix is very simple, just an else if not an if and I'm not willing to do anything more than the most simple!
• I've run my little bash script that does 20 iterations of the command run by the CI and they all come back the same - whereas before at least 5 or 6 were showing the random differences.

For the sake of the SBND CI there is a branch based on v09_93_01_p02 bugfix/hlay_mcs_invalid_read but this PR branch is rebased against develop.

I've added review requests from @gputnam and @wjdanswjddl as I see you have previous commits on this tool so may want to see this. The okay from Rachel or Joseph should be fine from the bugfix side of things though.

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• Does this PR affect CAF data format? If so, please assign a CAF maintainer as additional reviewer.
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[jzennamo]

Thank you @henrylay97! I look forward to the results of @RachelCoackley's CI test

[RachelCoackley]

Thanks for fixing this Henry!! :) These warnings no longer appeared in the CI tests