rdma: bugfix: ram_control_save_page()

We were not checking for a valid 'bytes_sent' pointer before accessing it.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael R. Hines <mrhines@us.ibm.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

savevm.c

@@ -662,7 +662,7 @@ size_t ram_control_save_page(QEMUFile *f, ram_addr_t block_offset,
                                     offset, size, bytes_sent);
 
         if (ret != RAM_SAVE_CONTROL_DELAYED) {
-            if (*bytes_sent > 0) {
+            if (bytes_sent && *bytes_sent > 0) {
                 qemu_update_position(f, *bytes_sent);
             } else if (ret < 0) {
                 qemu_file_set_error(f, ret);