Assign buf_ to nullptr in freeExtBuffer to mitigate damage from double free

Differential Revision: D21059558

fbshipit-source-id: 588e6ec2a6fdaf70ea5c03810358c84dedf81911

folly/io/IOBuf.cpp

@@ -1040,6 +1040,10 @@ void IOBuf::freeExtBuffer() noexcept {
   }
   SharedInfo::invokeAndDeleteEachObserver(
       observerListHead, [](auto& entry) { entry.afterFreeExtBuffer(); });
+
+#if FOLLY_MOBILE
+  buf_ = nullptr;
+#endif
 }
 
 void IOBuf::allocExtBuffer(