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

folly/io/IOBufQueue.cpp

@@ -46,11 +46,13 @@ void appendToChain(unique_ptr<IOBuf>& dst, unique_ptr<IOBuf>&& src, bool pack) {
       // joining two IOBufQueues together.
       size_t copyRemaining = MAX_PACK_COPY;
       std::size_t n;
-      while (src && (n = src->length()) < copyRemaining &&
-             n < tail->tailroom() && n > 0) {
-        memcpy(tail->writableTail(), src->data(), n);
-        tail->append(n);
-        copyRemaining -= n;
+      while (src && (n = src->length()) <= copyRemaining &&
+             n <= tail->tailroom()) {
+        if (n > 0) {
+          memcpy(tail->writableTail(), src->data(), n);
+          tail->append(n);
+          copyRemaining -= n;
+        }
         src = src->pop();
       }
     }