openvz: Check errors from virSocketAddrFormat

Commit id 'a4e86390' modified the command line to allow --ipadd multiple
times; however, it did not account for the condition where a NULL is
returned which will could lead to some interesting errors with multiple
--ipadd's without parameters.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/openvz/openvz_driver.c

@@ -910,6 +910,8 @@ openvzDomainSetNetwork(virConnectPtr conn, const char *vpsid,
         /* --ipadd ip */
         for (i = 0; i < net->nips; i++) {
             char *ipStr = virSocketAddrFormat(&net->ips[i]->address);
+            if (!ipStr)
+                goto cleanup;
             virCommandAddArgList(cmd, "--ipadd", ipStr, NULL);
             VIR_FREE(ipStr);
         }