Fix some crashes in imapx_command_start_next().

Start the next CamelIMAPXCommand before unreferencing the current one.

imapx_command_start() calls camel_imapx_command_close() on the current
command before initiating the next command, but imapx_command_start()
is being handed a potentially dangling CamelIMAPXCommand pointer.

This is most certainly causing of all the camel_imapx_command_close()
crashes, and likely a good number of memory corruption crashes as well.