mirror: fix uninitialized variable delay_ns warnings

The gcc 4.1.2 compiler warns that delay_ns may be uninitialized in
mirror_iteration().

There are two break statements in the do ... while loop that skip over
the delay_ns assignment.  These are probably the cause of the warning.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NBenoît Canet <benoit.canet@nodalink.com>

block/mirror.c

@@ -157,7 +157,7 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
     BlockDriverState *source = s->common.bs;
     int nb_sectors, sectors_per_chunk, nb_chunks;
     int64_t end, sector_num, next_chunk, next_sector, hbitmap_next_sector;
-    uint64_t delay_ns;
+    uint64_t delay_ns = 0;
     MirrorOp *op;
 
     s->sector_num = hbitmap_iter_next(&s->hbi);
@@ -247,8 +247,6 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
         next_chunk += added_chunks;
         if (!s->synced && s->common.speed) {
             delay_ns = ratelimit_calculate_delay(&s->limit, added_sectors);
-        } else {
-            delay_ns = 0;
         }
     } while (delay_ns == 0 && next_sector < end);