usnic: ensure to clean up nicely in case of low resources

If there are not enough resources (e.g., low VFs), we can end up
calling finalize_one_channel() on the same channel multiple times.  So
ensure to NULL out fields that we have freed already so that we do not
try to free them a second time.

Fixes CSCus26648.

opal/mca/btl/usnic/btl_usnic_module.c

@@ -1942,6 +1942,7 @@ static void finalize_one_channel(opal_btl_usnic_module_t *module,
 
     if (NULL != channel->info) {
         fi_freeinfo(channel->info);
+        channel->info = NULL;
     }
 
     /* gets set right after constructor called, lets us know recv_segs
@@ -2326,6 +2327,11 @@ static int init_channels(opal_btl_usnic_module_t *module)
     struct fi_av_attr av_attr;
     struct fi_eq_attr eq_attr;
 
+    memset(&module->mod_channels[0], 0,
+           sizeof(module->mod_channels[0]));
+    memset(&module->mod_channels[1], 0,
+           sizeof(module->mod_channels[1]));
+
     memset(&av_attr, 0, sizeof(av_attr));
     av_attr.type = FI_AV_MAP;
     av_attr.flags = FI_EVENT;