qemu_driver: avoid NULL dereference

* src/qemu/qemu_driver.c (qemudDomainStart): After setting vm to NULL,
goto cleanup, rather than dereferencing the NULL pointer.

src/qemu/qemu_driver.c

@@ -6489,8 +6489,11 @@ static int qemudDomainStart(virDomainPtr dom) {
          * We should still have a reference left to vm but
          * one should check for 0 anyway
          */
-        if (qemuDomainObjEndJob(vm) == 0)
+        if (qemuDomainObjEndJob(vm) = 0) {
             vm = NULL;
+            goto cleanup;
+        }
+
         virDomainObjUnlock(vm);
         qemuDriverUnlock(driver);
         ret = qemudDomainRestore(dom->conn, managed_save);