assert in ~WebContentsDelegateQt when loading invalid URL and interceptor is set

How to reproduce:

• create a new QWebEngineUrlRequestInterceptor
• just return true or false from interceptRequest.
• set this interceptor on the current profile
• load an invalid URL

When destroying QWebEnginePage, the destructor of WebContentsDelegateQt will assert:

ASSERT: "m_loadingErrorFrameList.isEmpty()" in file /home/jobor/dev/qt/5.6/qtwebengine/src/core/web_contents_delegate_qt.h, line 69

To be seen in the last block of tst_QWebEngineUrlRequestInterceptor::interceptRequest().