sheepdog: check if '-o redundancy' is passed from user

This fix a segfault (that is caused by b3af018f) of following command:

$ qemu-img convert some_img sheepdog:some_img

Cc: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NLiu Yuan <namei.unix@gmail.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/sheepdog.c

@@ -1666,9 +1666,11 @@ static int sd_create(const char *filename, QEMUOptionParameter *options,
                 goto out;
             }
         } else if (!strcmp(options->name, BLOCK_OPT_REDUNDANCY)) {
-            ret = parse_redundancy(s, options->value.s);
-            if (ret < 0) {
-                goto out;
+            if (options->value.s) {
+                ret = parse_redundancy(s, options->value.s);
+                if (ret < 0) {
+                    goto out;
+                }
             }
         }
         options++;