xen-blkback: fix foreach_grant_safe to handle empty lists
xen-blkback: fix foreach_grant_safe to handle empty lists

We may use foreach_grant_safe in the future with empty lists, so make
sure we can handle them.

Signed-off-by: Roger Pau Monné <1c35b25b17252e1e5021df00666589fc74f64580@citrix.com>
Cc: 196a79ea1bad81d8c954adf793448b0a45442f28@lists.xen.org
Cc: Konrad Rzeszutek Wilk <da3a51b335cef0eb0e2c329c5ef6bcd6acef687a@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <da3a51b335cef0eb0e2c329c5ef6bcd6acef687a@oracle.com>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs