xnu: Fix double free in grub_xnu_devprop_add_property()
    
    grub_xnu_devprop_add_property() should not free utf8 and utf16 as it get
    allocated and freed in the caller.
    
    Minor improvement: do prop fields initialization after memory allocations.
    
    Fixes: CID 292442, CID 292457, CID 292460, CID 292466
    
    Signed-off-by: Alexey Makhalov <amakhalov@vmware.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>