Fix infer issues - USE_AFTER_DELETE

[eljonny]

"I'm 99.9% sure this is a false positive."

Definitely not a false positive, and pointed out 2 other instances of the issue.

src/TestCPPTestCase.cpp:203: error: Use After Delete
  call to `std::unique_ptr::operator=()` eventually accesses `TestCPP::Util::no_destroy::get().__infer_backing_pointer` that was invalidated by `delete` on line 202.
  201.                 cout.rdbuf(TestCase::stdoutOriginal.get().release());
  202.                 delete TestCase::stdoutBuffer.get().get();
  203.                 TestCase::stdoutBuffer.get() = nullptr;
                       ^
  204.             }
  205.

[eljonny]

This could potentially cause a double-delete; setting the unique_ptr value to nullptr calls delete, so I'm doing extra work here that doesn't need to happen.

Reference: here and, more specifically (and officially) here.

Basically, assigning nullptr to be the value of a unique_ptr is defined specifically in the standard at the specified location and causes the default deleter to be called via a call to std::unique_ptr<U, E>::reset().

That section specifies the following operator=(nullptr_t) for std::unique_ptr<U, E>:

    unique_ptr& operator=(nullptr_t) noexcept;

 8      Effects: reset().
 9      Postcondition: get() == nullptr
10      Returns: *this.

And, by definition, std::unique_ptr<U, E>::reset() states the following:

    void reset(pointer p = pointer()) noexcept;

 3      Requires: The expression get_deleter()(get()) shall be well formed, shall have well-defined behavior, and shall not throw exceptions.
 4      Effects: assigns p to the stored pointer, and then if the old value of the stored pointer, old_p, was notequal to nullptr, calls get_deleter()(old_p). [ Note: The order of these operations is significant because the call to get_deleter() may destroy *this. — end note ]
 5      Postconditions: get() == p. [ Note: The postcondition does not hold if the call to get_deleter() destroys *this since this->get() is no longer a valid expression. — end note ]

Later standards may alter this behavior or it may be in a different location in the standard document (for instance in C++14 it is in section 20.9.1.2.3 and 20.9.1.2.5); I am following C++11 (document n3242, JTC1, SC22, WG21) currently.