MINOR: buffer: reset a buffer in b_reset() and not channel_init()

We'll soon need to be able to switch buffers without touching the
channel, so let's move buffer initialization out of channel_init().
We had the same in compressoin.c.
diff --git a/src/session.c b/src/session.c
index 7723074..d57a2d5 100644
--- a/src/session.c
+++ b/src/session.c
@@ -488,6 +488,7 @@
 
 	/* initialize the request buffer */
 	s->req->buf->size = global.tune.bufsize;
+	b_reset(s->req->buf);
 	channel_init(s->req);
 	s->req->prod = &s->si[0];
 	s->req->cons = &s->si[1];
@@ -505,6 +506,7 @@
 
 	/* initialize response buffer */
 	s->rep->buf->size = global.tune.bufsize;
+	b_reset(s->rep->buf);
 	channel_init(s->rep);
 	s->rep->prod = &s->si[1];
 	s->rep->cons = &s->si[0];