qemuBackupBegin: Don't leak 'def' on early failures

The cleanup path expects that 'def' is assigned to 'priv->backup', but
that's not the case for early failures. Add a check to stop overwriting
of 'def' so that it can be freed.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_backup.c

@@ -806,7 +806,7 @@ qemuBackupBegin(virDomainObjPtr vm,
         ignore_value(qemuDomainObjExitMonitor(priv->driver, vm));
     }
 
-    if (ret < 0 && !job_started)
+    if (ret < 0 && !job_started && priv->backup)
         def = g_steal_pointer(&priv->backup);
 
     if (ret == 0)