input_plugin: add method check()

To check for errors without reading.  The decoder thread wants to do
that, before it passes the input stream to the plugin.

test/run_input.c

@@ -100,6 +100,12 @@ dump_input_stream(struct input_stream *is)
 			break;
 	}
 
+	if (!input_stream_check(is, &error)) {
+		g_warning("%s", error->message);
+		g_error_free(error);
+		return EXIT_FAILURE;
+	}
+
 	return 0;
 }