block/mirror: fix -Werror=maybe-uninitialized false-positive

../block/mirror.c:1066:22: error: ‘iostatus’ may be used uninitialized [-Werror=maybe-uninitialized]

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
diff --git a/block/mirror.c b/block/mirror.c
index 61f0a71..54e3a7e 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -931,7 +931,7 @@
     MirrorBDSOpaque *mirror_top_opaque = s->mirror_top_bs->opaque;
     BlockDriverState *target_bs = blk_bs(s->target);
     bool need_drain = true;
-    BlockDeviceIoStatus iostatus;
+    BlockDeviceIoStatus iostatus = BLOCK_DEVICE_IO_STATUS__MAX;
     int64_t length;
     int64_t target_length;
     BlockDriverInfo bdi;