uri: don't rebase null views

uri.cpp

@@ -205,7 +205,9 @@ void uri::clear_fragment ()
         m_views[FRAGMENT].begin () - m_value.data (),
         m_views[FRAGMENT].size ()
     );
-    m_views[FRAGMENT] = nullptr;
+    // Don't set this to nullptr. Other code assumes these views aren't null
+    // (eg, for offsetting during copy construction).
+    m_views[FRAGMENT] = { m_views[FRAGMENT].begin (), 0 };
 }