block/get_block_status: set *pnum = 0 on error

if the call is invoked through bdrv_is_allocated the caller might
expect *pnum = 0 on error. however, a new implementation of
bdrv_get_block_status might only return a negative exit value on
error while keeping *pnum untouched.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -3162,6 +3162,7 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
 
     ret = bs->drv->bdrv_co_get_block_status(bs, sector_num, nb_sectors, pnum);
     if (ret < 0) {
+        *pnum = 0;
         return ret;
     }