plugin: fix feature_set allocation

Using `tal_dup_or_null` does not set the tal context to the included
bytes array. Luckily we already have a function tha deeply duplicates
a `feature_set`: `feature_set_dup`

Signed-off-by: Peter Neuroth <[email protected]>

Author: nepet

lightningd/plugin.c

@@ -1821,7 +1821,7 @@ static const char *plugin_parse_getmanifest_response(const char *buffer,
 		}
 
 		/* Store fset to allow to remove feature bits when init returns disabled */
-		plugin->fset = tal_dup_or_null(plugin, struct feature_set, fset);
+		plugin->fset = feature_set_dup(plugin, fset);
 	} else {
 		plugin->fset = NULL;
 	}