zfs: Resolve RESOURCE_LEAK

Found by my Coverity checker - virCheckFlags call could return -1, but
not virCommandFree(destroy_cmd).
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/storage/storage_backend_zfs.c

@@ -355,10 +355,12 @@ virStorageBackendZFSDeleteVol(virConnectPtr conn ATTRIBUTE_UNUSED,
                               unsigned int flags)
 {
     int ret = -1;
-    virCommandPtr destroy_cmd = virCommandNewArgList(ZFS, "destroy", NULL);
+    virCommandPtr destroy_cmd = NULL;
 
     virCheckFlags(0, -1);
 
+    destroy_cmd = virCommandNewArgList(ZFS, "destroy", NULL);
+
     virCommandAddArgFormat(destroy_cmd, "%s/%s",
                            pool->def->source.name, vol->name);