[UI] Include null terminator in MessageBoxDialog

src/xenia/ui/imgui_dialog.cc

@@ -67,7 +67,7 @@ class MessageBoxDialog final : public ImGuiDialog {
                                ImGuiWindowFlags_AlwaysAutoResize)) {
       char* text = const_cast<char*>(body_.c_str());
       ImGui::InputTextMultiline(
-          "##body", text, body_.size(), ImVec2(600, 0),
+          "##body", text, body_.size() + 1, ImVec2(600, 0),
           ImGuiInputTextFlags_AutoSelectAll | ImGuiInputTextFlags_ReadOnly);
       if (ImGui::Button("OK")) {
         ImGui::CloseCurrentPopup();