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/proto_http.c b/src/proto_http.c
index 7f90373..88cb636 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -6158,7 +6158,7 @@
 		if ((s->req.flags & (CF_SHUTR|CF_SHUTW)) == (CF_SHUTR|CF_SHUTW))
 			goto aborted_xfer;
 		/* If we have some pending data, we continue the processing */
-		if (!buffer_pending(res->buf)) {
+		if (!ci_data(res)) {
 			if (!(s->flags & SF_ERR_MASK))
 				s->flags |= SF_ERR_SRVCL;
 			HA_ATOMIC_ADD(&s->be->be_counters.srv_aborts, 1);