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

rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly