Handle `memory` being `None` when `realloc` requires it.

[michael-weigelt]

The spec 1, 2 states that if the canonopt realloc is given, then the canonopt memory is required. In the 64bit component model, the dependency is obvious because the signature of realloc depends on the address type of memory.

Currently, all address types are assumed to be i32, and at least the realloc implementation does not require a memory to be passed. Requiring it may be a breaking change.

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