libxl: check return value of libxl__xs_directory in name2bdf

libxl: check return value of libxl__xs_directory in name2bdf

libxl__xs_directory() can potentially return NULL without setting `n`.
As `n` isn't initialised, we need to check libxl__xs_directory()
return value before checking `n`. Otherwise, `n` might be non-zero
with `bdfs` NULL which would lead to a segv.

Fixes: 57bff091f4 ("libxl: add 'name' field to 'libxl_device_pci' in the IDL...")
Reported-by: "G.R." <firemeteor@users.sourceforge.net>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-by: "G.R." <firemeteor@users.sourceforge.net>