[PLAT-11320] Fix an off-by-1 error when fetching arm64 register values that could potentially run off the end of the array

[kstenerud]

Goal

Fix an off-by-1 error when fetching arm64 register values that could potentially run off the end of the array. This was only off-by-1 and only reading, so in the worst case it would fetch bogus data for the register requested. We don't actually fetch the upper registers, but best to be safe regardless.

Testing

We never fetch the upper registers, so we don't really have anything to test, other than that it still fetches the registers we are interested in.

The existing tests make sure the register fetches still work post-fix.

[github-actions]

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