fix IOBuf self move-assignment

Summary: Properly handle `this == &other` case.

Test Plan: fbconfig -r folly/test && fbmake runtests_opt -j32

Reviewed By: simpkins@fb.com

FB internal diff: D1314916

folly/io/IOBuf.cpp

@@ -365,6 +365,10 @@ IOBuf::~IOBuf() {
 }
 
 IOBuf& IOBuf::operator=(IOBuf&& other) noexcept {
+  if (this == &other) {
+    return *this;
+  }
+
   // If we are part of a chain, delete the rest of the chain.
   while (next_ != this) {
     // Since unlink() returns unique_ptr() and we don't store it,