loader/xnu: Fix memory leak

loader/xnu: Fix memory leak

In grub_xnu_load_kext_from_dir(), when the call to grub_device_open()
failed, it simply cleaned up previously allocated memory and returned
GRUB_ERR_NONE. However, it neglected to free ctx->newdirname which is
allocated before the call to grub_device_open().

Fixes: CID 473859
Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>