libxl: Fix uninitialized pointer when passing an empty cdrom

libxl: Fix uninitialized pointer when passing an empty cdrom

Commit 3fec17d4bb56567d139d7806392f4d8702d3f6a7 introduced a bug where
an empty cdrom would cause target_path to be uninitialized.  Initialize
target_path to NULL instead.

The other option here would have been to set target_path to NULL only
on the LIBXL_DISK_FORMAT_EMPTY path.  That would potentially enable
the compiler to catch future uninitialized paths, rather than having
those paths (potentially) dereference a NULL pointer.  But given that
a bunch of our compilers failed to catch *this* uninitialized path,
setting it to NULL at declaration seems the safer option for now.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>