usnic: ensure to free all resources if no usnic BTLs found

If all usnic devices are excluded, then we need to ensure the error
path includes freeing the filter.

This was Coverity CID 1288085

opal/mca/btl/usnic/btl_usnic_component.c

@@ -598,7 +598,7 @@ static mca_btl_base_module_t** usnic_component_init(int* num_btl_modules,
     int i, j, num_final_modules;
     int num_devs;
     opal_btl_usnic_module_t *module;
-    usnic_if_filter_t *filter;
+    usnic_if_filter_t *filter = NULL;
     bool keep_module;
     bool filter_incl = false;
     int min_distance, num_local_procs;
@@ -722,8 +722,6 @@ static mca_btl_base_module_t** usnic_component_init(int* num_btl_modules,
 
         filter_incl = false;
         filter = parse_ifex_str(mca_btl_usnic_component.if_exclude, "exclude");
-    } else {
-        filter = NULL;
     }
 
     num_local_procs = opal_process_info.num_local_peers;
@@ -1000,6 +998,13 @@ static mca_btl_base_module_t** usnic_component_init(int* num_btl_modules,
     mca_btl_usnic_component.usnic_all_modules = NULL;
     free(mca_btl_usnic_component.usnic_active_modules);
     mca_btl_usnic_component.usnic_active_modules = NULL;
+
+    /* free filter if created */
+    if (filter != NULL) {
+        free_filter(filter);
+        filter = NULL;
+    }
+
     goto send_modex;
 }