Slight optimization of append logic
Slight optimization of append logic

Summary:
1) There's no need to exit the loop if we encounter and IOBuf of length 0.
2) We should do `n <= tailroom()` instead of `n < tailroom()`.

Reviewed By: yfeldblum

Differential Revision: D14715582

fbshipit-source-id: 9a30454bd536929414f7c9013dbcf6e59e6e7f2c

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