Be stricter about changing to the same state in wxWebRequest

This shouldn't happen, and doesn't, but if it ever does we'd better
return immediately rather than doing something that will almost surely
be wrong.

src/common/webrequest.cpp

@@ -205,7 +205,7 @@ struct StateEventProcessor
 
 void wxWebRequestImpl::SetState(wxWebRequest::State state, const wxString & failMsg)
 {
-    wxASSERT_MSG( state != m_state, "shouldn't switch to the same state" );
+    wxCHECK_RET( state != m_state, "shouldn't switch to the same state" );
 
     wxLogTrace(wxTRACE_WEBREQUEST, "Request %p: state %d => %d", this, m_state, state);