Prevent reading beyond bounds of p_access_chain.

[kuwerty]

valgrind threw this out and it does seem like an invalid access. There's a bounds check on index_index present but it's after the dereference so I'm just guessing at this fix.

[CLAassistant]

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[chaoticbob]

Hi - can you sign the CLA?

[kuwerty]

Sure, it's signed.