golang/xenlight: Don't try to marshall zero-length arrays in fromC

golang/xenlight: Don't try to marshall zero-length arrays in fromC

The current fromC array code will do the "magic" casting and
martialling even when num_foo variable is 0.  Go crashes when doing
the cast.

Only do array marshalling if the number of elements is non-zero;
otherwise, leave the target pointer empty (nil for Go slices, NULL for
C arrays).

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Nick Rosbrook <rosbrookn@ainfosec.com>