Support building on GCC 6.3

Summary:
This is a little strange: we already build with the system GCC on Debian 9 (Stretch),
which is GCC 6.3 - but these changes are needed on other distributions when building GCC 6.3
verbatim from upstream.

fixes facebook/folly#823

Reviewed By: jjergus

Differential Revision: D17533036

fbshipit-source-id: 32f7d1dc1fb9af3a89fe628185c0b239203e86d5

folly/io/IOBufQueue.h

@@ -53,14 +53,14 @@ class IOBufQueue {
 
     WritableRangeCacheData(WritableRangeCacheData&& other)
         : cachedRange(other.cachedRange), attached(other.attached) {
-      other.cachedRange = {};
+      other.cachedRange = {nullptr, nullptr};
       other.attached = false;
     }
     WritableRangeCacheData& operator=(WritableRangeCacheData&& other) {
       cachedRange = other.cachedRange;
       attached = other.attached;
 
-      other.cachedRange = {};
+      other.cachedRange = {nullptr, nullptr};
       other.attached = false;
 
       return *this;