fix invalid-null-argument UBSAN error in IOBufQueue.cpp

Summary:
UndefinedBehaviorSanitizer: invalid-null-argument folly/io/IOBufQueue.cpp:53:38
```

Add a check that the length is greater than zero.

Reviewed By: meyering

Differential Revision: D7139027

fbshipit-source-id: 37585092429af52d14c98c193ecc495752bbda1c

folly/io/IOBufQueue.cpp

@@ -47,9 +47,8 @@ appendToChain(unique_ptr<IOBuf>& dst, unique_ptr<IOBuf>&& src, bool pack) {
       // joining two IOBufQueues together.
       size_t copyRemaining = MAX_PACK_COPY;
       uint64_t n;
-      while (src &&
-             (n = src->length()) < copyRemaining &&
-             n < tail->tailroom()) {
+      while (src && (n = src->length()) < copyRemaining &&
+             n < tail->tailroom() && n > 0) {
         memcpy(tail->writableTail(), src->data(), n);
         tail->append(n);
         copyRemaining -= n;