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>

tools/libxc/xc_private.c

@@ -198,6 +198,9 @@ static int xc_interface_close_common(xc_interface *xch)
 {
     int rc = 0;
 
+    if (!xch)
+	return 0;
+
     xc__hypercall_buffer_cache_release(xch);
 
     xtl_logger_destroy(xch->dombuild_logger_tofree);