Fix concat_hstring for 0-length string (#675)

Author: oldnewthing

strings/base_string_operators.h

@@ -98,7 +98,12 @@ namespace winrt::impl
 {
     inline hstring concat_hstring(std::wstring_view const& left, std::wstring_view const& right)
     {
-        hstring_builder text(static_cast<uint32_t>(left.size() + right.size()));
+        auto size = static_cast<uint32_t>(left.size() + right.size());
+        if (size == 0)
+        {
+            return{};
+        }
+        hstring_builder text(size);
         memcpy_s(text.data(), left.size() * sizeof(wchar_t), left.data(), left.size() * sizeof(wchar_t));
         memcpy_s(text.data() + left.size(), right.size() * sizeof(wchar_t), right.data(), right.size() * sizeof(wchar_t));
         return text.to_hstring();

test/old_tests/UnitTests/hstring.cpp

@@ -570,4 +570,7 @@ TEST_CASE("hstring, concat")
     REQUIRE(hstring() + s == L"abc");
     REQUIRE(s + L"" == L"abc");
     REQUIRE(L"" + s == L"abc");
+
+    REQUIRE(hstring() + hstring() == L"");
+    REQUIRE(get_abi(hstring() + hstring()) == nullptr);
 }