Fix uninitialized WFIFO/RFIFO buffer memory on realloc

[MrKeiKun]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

• realloc_fifo(): zero the newly added tail of rdata/wdata after each RECREATE, when the new size is larger than the old one.
• realloc_writefifo(): same, but only on the grow path (the shrink path frees no new memory, so nothing to zero there).
• Both are guarded against a hypothetical shrink-sized "grow" call to avoid a size_t underflow in the memset length.

Issues addressed: #1743