plug-ins: Fix Python console crash after clear

GTK2 get_iter_at_line_offset() does not allow invalid offsets unlike
GTK3 version. Clearing the console creates an invalid iter due to an
empty buffer.

This patch attempts to resolve it by adding a ">>> " output when the
console is cleared, thus preventing the invalid iter issue.

plug-ins/pygimp/plug-ins/pyconsole.py

@@ -569,6 +569,7 @@ class _Console(_ReadLine, code.InteractiveInterpreter):
 
         self.__start()
         self.run_on_raw_input = start_script
+        self.raw_input(self.ps1)
 
     def do_raw_input(self, text):
         if self.cmd_buffer: