MEDIUM: mux: make mux->rcv_buf() take a size_t for the count

It also returns a size_t. This is in order to clean the API. Note
that the H2 mux still uses some ints in the functions called from
h2_rcv_buf(), though it's not really a problem given that H2 frames
are smaller. It may deserve a general cleanup later though.
diff --git a/src/mux_pt.c b/src/mux_pt.c
index d65c4e2..aa03fd4 100644
--- a/src/mux_pt.c
+++ b/src/mux_pt.c
@@ -159,16 +159,16 @@
 /*
  * Called from the upper layer, to get more data
  */
-static int mux_pt_rcv_buf(struct conn_stream *cs, struct buffer *buf, int count)
+static size_t mux_pt_rcv_buf(struct conn_stream *cs, struct buffer *buf, size_t count)
 {
-	int ret;
+	size_t ret;
 
 	ret = cs->conn->xprt->rcv_buf(cs->conn, buf, count);
 	if (conn_xprt_read0_pending(cs->conn))
 		cs->flags |= CS_FL_EOS;
 	if (cs->conn->flags & CO_FL_ERROR)
 		cs->flags |= CS_FL_ERROR;
-	return (ret);
+	return ret;
 }
 
 /* Called from the upper layer, to send data */