virExecWithHook: avoid leak on OOM error path

* src/util/util.c (virExecWithHook): Free argv_str string before
returning upon failure to allocate space for environment.

src/util/util.c

@@ -631,6 +631,7 @@ virExecWithHook(virConnectPtr conn,
 
     if (envp) {
         if ((envp_str = virArgvToString(envp)) == NULL) {
+            VIR_FREE(argv_str);
             virReportOOMError(conn);
             return -1;
         }