btl/openib: fix a double free

as reported by Coverity with CID 1287033

opal/mca/btl/openib/btl_openib_component.c

@@ -1853,6 +1853,7 @@ static int init_one_device(opal_list_t *btl_list, struct ibv_device* ib_dev)
         }
     }
     free(allowed_ports);
+    allowed_ports = NULL;
 
     /* If we made a BTL, check APM status and return.  Otherwise, fall
        through and destroy everything */