Fix when building with -Os:

    CC      block/block-copy.o
  block/block-copy.c: In function ‘block_copy_task_entry’:
  block/block-copy.c:428:38: error: ‘error_is_read’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
    428 |         t->call_state->error_is_read = error_is_read;
        |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200507121129.29760-2-philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>