MINOR: buffer: replace buffer_pending() with ci_data()

It used to return b->i for channels, which is what ci_data() does.
diff --git a/src/filters.c b/src/filters.c
index 26bed64..69a0331 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -1094,7 +1094,7 @@
 	/* Stop waiting data if the input in closed and no data is pending or if
 	 * the output is closed. */
 	if ((chn->flags & CF_SHUTW) ||
-	    ((chn->flags & CF_SHUTR) && !buffer_pending(chn->buf))) {
+	    ((chn->flags & CF_SHUTR) && !ci_data(chn))) {
 		ret = 1;
 		goto end;
 	}