libxenctrl: Fix xc_interface_close() crash if it gets NULL as an argument

libxenctrl: Fix xc_interface_close() crash if it gets NULL as an argument

xc_interface_close() crashes if it gets NULL as an argument. However,
it just calls xc_interface_close_common() which is called by many
others functions. It means that they are also vulnerable. So fix above
mentioned issue by adding NULL check in xc_interface_close_common().
This way we fix similar issue in other functions which calls
xc_interface_close_common() too.

Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>