global: start relying on [io]_stream_unref(NULL) being a no-op

Cleanup performed with the following semantic patch:

	@@
	expression E;
	@@

	- if (E != NULL) {
	- 	i_stream_unref(&E);
	- }
	+ i_stream_unref(&E);

	@@
	expression E;
	@@

	- if (E != NULL) {
	- 	o_stream_unref(&E);
	- }
	+ o_stream_unref(&E);