Fix coverity defects: CID 153459

CID 153459: Null pointer dereferences (FORWARD_NULL)
Accidentally introduced by #5159.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: luozhengzheng <luo.zhengzheng@zte.com.cn>
Closes #5310

module/zfs/zfs_fm.c

@@ -219,7 +219,7 @@ zfs_ereport_start(nvlist_t **ereport_out, nvlist_t **detector_out,
 	}
 
 	if ((strcmp(subclass, FM_EREPORT_ZFS_DELAY) == 0) &&
-	    !zio->io_timestamp) {
+	    (zio != NULL) && (!zio->io_timestamp)) {
 		/* Ignore bogus delay events */
 		return;
 	}