Add default constructor to QuicBuffer

Summary:
This diff adds a default constructor to QuicBuffer that behaves similarly to folly::IOBuf's default constructor.

This is used in `InplaceQuicPacketBuilder::buildPacket` where the else branch is the fallback for cases where no header/body spans were established (e.g. the builder never wrote out the header or payload), so we release the borrowed buffer and return a packet with default-constructed Buf()s.

Reviewed By: jbeshay

Differential Revision: D85726178

fbshipit-source-id: 249cb3042052221a09afc9230a87d4bea34e07d3

Author: Aman Sharma

quic/common/QuicBuffer.h

@@ -24,6 +24,8 @@ class QuicBuffer {
     CREATE = 0,
   };
 
+  QuicBuffer() : next_(this), prev_(this) {}
+
   explicit QuicBuffer(std::size_t capacity);
 
   // Same as the previous constructor, just added the CreateOp