Zero-fill padding in detach_abi(com_array) (#1165)

A code analysis warning recently fired for a customer on detach_abi(com_array<T>). This function returns a std::pair<uint32_t, some_pointer_type>, which will have, on 64-bit builds, 4 bytes of padding between the uint32 size and the pointer members. Currently, that padding is uninitialized.

The idea behind the code analysis warning is that information may be leaked via those unitialized bytes.

In practice, that's almost never going to be an issue for this function, because the std::pair is not an interesting object to pass around, and only exists as a convenience to return both the size and buffer of the com_array at the same time.

However, the fix removes a pain point for a customer, is simple, risk-free, and actually gets optimized away in the 99% use case (return value stored in a local variable, access only the first/second members, not the padding bytes).  Demo showing optimization: https://godbolt.org/z/T4vPhMKxn

strings/base_array.h

@@ -420,7 +420,10 @@ WINRT_EXPORT namespace winrt
     template <typename T>
     auto detach_abi(com_array<T>& object) noexcept
     {
-        std::pair<uint32_t, impl::arg_out<T>> result(object.size(), *reinterpret_cast<impl::arg_out<T>*>(&object));
+        std::pair<uint32_t, impl::arg_out<T>> result;
+        memset(&result, 0, sizeof(result));
+        result.first = object.size();
+        result.second = *reinterpret_cast<impl::arg_out<T>*>(&object);
         memset(&object, 0, sizeof(com_array<T>));
         return result;
     }