BUG/MEDIUM: buffers: fix bi_putchr() to correctly advance the pointer

bi_putchr() failed to move the buffer pointer forward. The only user
was the peer handler which was broken, it failed to sync. Thanks to
Hervé Commowick for reporting the issue.
diff --git a/src/buffers.c b/src/buffers.c
index 7becb48..6eeb64f 100644
--- a/src/buffers.c
+++ b/src/buffers.c
@@ -154,9 +154,7 @@
 	if (buf->to_forward >= 1) {
 		if (buf->to_forward != BUF_INFINITE_FORWARD)
 			buf->to_forward--;
-		buf->o++;
-		buf->i--;
-		buf->flags &= ~BF_OUT_EMPTY;
+		b_adv(buf, 1);
 	}
 
 	buf->total++;