input_stream: let the implementation assign is->plugin

This way, plugins can manipulate the plugin pointer during open().

src/input_archive.c

@@ -87,6 +87,7 @@ input_archive_open(struct input_stream *is, const char *pathname)
 	is->plugin = &input_plugin_archive;
 
 	//internal handle
+	is->plugin = &input_plugin_archive;
 	is->data = arch_ctx;
 
 	//open archive

src/input_curl.c

@@ -909,6 +909,7 @@ input_curl_open(struct input_stream *is, const char *url)
 	c->buffers = g_queue_new();
 	c->rewind = g_queue_new();
 
+	is->plugin = &input_plugin_curl;
 	is->data = c;
 
 	c->multi = curl_multi_init();

src/input_file.c

@@ -62,6 +62,7 @@ input_file_open(struct input_stream *is, const char *filename)
 	posix_fadvise(fd, (off_t)0, is->size, POSIX_FADV_SEQUENTIAL);
 #endif
 
+	is->plugin = &input_plugin_file;
 	is->data = GINT_TO_POINTER(fd);
 	is->ready = true;
 

src/input_stream.c

@@ -80,7 +80,13 @@ input_stream_open(struct input_stream *is, const char *url)
 		const struct input_plugin *plugin = input_plugins[i];
 
 		if (plugin->open(is, url)) {
-			is->plugin = plugin;
+			assert(is->plugin != NULL);
+			assert(is->plugin->open == NULL ||
+			       is->plugin == plugin);
+			assert(is->plugin->close != NULL);
+			assert(is->plugin->read != NULL);
+			assert(is->plugin->eof != NULL);
+
 			return true;
 		}
 	}