qemu: Avoid dereference of NULL pointer

If something fails while initializing qemu job object in
qemuDomainObjPrivateAlloc(), memory to the private pointer is freed, but
after that, the pointer is still dereferenced, which may result in a
segfault.

* qemuDomainObjPrivateAlloc() - Don't dereference NULL pointer.

src/qemu/qemu_domain.c

@@ -214,11 +214,15 @@ static void *qemuDomainObjPrivateAlloc(void)
         return NULL;
 
     if (qemuDomainObjInitJob(priv) < 0)
-        VIR_FREE(priv);
+        goto error;
 
     priv->migMaxBandwidth = QEMU_DOMAIN_DEFAULT_MIG_BANDWIDTH_MAX;
 
     return priv;
+
+error:
+    VIR_FREE(priv);
+    return NULL;
 }
 
 static void qemuDomainObjPrivateFree(void *data)