Don't crash in wxOSX::wxClipboard::Clear() if initialization failed.

Creating the clipboard may fail (e.g. when running from a ssh session to an OS
X machine), don't crash by passing NULL pointer to PasteboardClear() if this
happens but assert and return instead.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65383 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/carbon/clipbrd.cpp

@@ -63,6 +63,8 @@ void wxClipboard::Clear()
 {
     wxDELETE(m_data);
 
+    wxCHECK_RET( m_pasteboard, "Clipboard creation failed." );
+
     OSStatus err = PasteboardClear( m_pasteboard );
     if (err != noErr)
     {