src: add fast path for equal size to `Reallocate()`
When old and new size match, we can skip the rest of the function, which makes sense in the case of embedders who do not use Node's allocator, as that would lead to needlessly allocating and freeing buffers of identical sizes.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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