libxl: Avoid realloc(,0) when libxl__xs_directory returns empty list

libxl: Avoid realloc(,0) when libxl__xs_directory returns empty list

If the named path is a leaf node, libxl__xs_directory can succeed,
returning non-null, but set *nb to 0.

In three places in libxl this may result in a zero size argument being
passed to malloc() or realloc(), which is not adviseable.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>