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;

interface/wx/webrequest.h

@@ -187,6 +187,9 @@ public:
 
         Events will be triggered on success or failure.
 
+        The current state must be @c State_Idle, already started requests can't
+        be started again.
+
         @see Cancel()
     */
     void Start();

src/common/webrequest.cpp

@@ -391,6 +391,9 @@ void wxWebRequest::Start()
 {
     wxCHECK_IMPL_VOID();
 
+    wxCHECK_RET( m_impl->GetState() == wxWebRequest::State_Idle,
+                 "Completed requests can not be restarted" );
+
     m_impl->Start();
 }
 

src/common/webrequest_curl.cpp

@@ -187,9 +187,6 @@ wxWebRequestCURL::~wxWebRequestCURL()
 
 void wxWebRequestCURL::Start()
 {
-    if ( GetState() != wxWebRequest::State_Idle )
-        return;
-
     m_response.reset(new wxWebResponseCURL(*this));
 
     if ( m_dataSize )

src/msw/webrequest_winhttp.cpp

@@ -237,9 +237,6 @@ void wxWebRequestWinHTTP::SetFailed(DWORD errorCode)
 
 void wxWebRequestWinHTTP::Start()
 {
-    if ( GetState() != wxWebRequest::State_Idle ) // Completed requests can not be restarted
-        return;
-
     // Parse the URL
     wxURI uri(m_url);
     bool isSecure = uri.GetScheme().IsSameAs("HTTPS", false);