Ensure wxWebRequest is in idle state before starting it

Check that current state is State_Idle in wxWebRequest itself only once
instead of doing it in 2 (out of 3) wxWebRequestImpl implementations.

Also assert if this is not the case instead of silently doing nothing
which would surely be more difficult to debug.

include/wx/private/webrequest.h

@@ -64,6 +64,7 @@ public:
 
     wxWebRequest::Storage GetStorage() const { return m_storage; }
 
+    // Precondition for this method checked by caller: current state is idle.
     virtual void Start() = 0;
 
     virtual void Cancel() = 0;