Add return statement to a test class's assignment operator. Defect found by Coverity Scan.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216317 91177308-0d34-0410-b5e6-96231b3b80d8

test/utilities/memory/unique.ptr/unique.ptr.runtime/unique.ptr.runtime.ctor/default01.fail.cpp

@@ -25,7 +25,7 @@ class Deleter
 public:
 
     Deleter(Deleter&) {}
-    Deleter& operator=(Deleter&) {}
+    Deleter& operator=(Deleter&) { return *this; }
 
     void operator()(void*) const {}
 };