Change type of allocate64 to uint64_t (backport #9307)

[mergify]

The claim in #3526 that there could be an integer overflow is not true on 64-bit architectures. It could only happen if size_t is 32-bits. The solution is to change the type of allocate64 to uint64_t so that it isn't affected by the architecture. I was puzzled why the variable is called allocate64 but its type is size_t. It turns out that this is a regression that happened in 7576a27. I had previously changed the type to uint64_t in e4e3362.

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This is an automatic backport of pull request #9307 done by Mergify.

[mergify]

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