MINOR: buffer: remove bo_end()

It was replaced by either b_tail() when the buffer has no input data, or
b_peek(b, b->o).
diff --git a/src/mux_h2.c b/src/mux_h2.c
index afd33cb..5a7af54 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -2988,7 +2988,7 @@
 	chunk_reset(&outbuf);
 
 	while (1) {
-		outbuf.str  = bo_end(h2c->mbuf);
+		outbuf.str  = b_tail(h2c->mbuf);
 		outbuf.size = bo_contig_space(h2c->mbuf);
 		outbuf.len = 0;
 
@@ -3146,7 +3146,7 @@
 	chunk_reset(&outbuf);
 
 	while (1) {
-		outbuf.str  = bo_end(h2c->mbuf);
+		outbuf.str  = b_tail(h2c->mbuf);
 		outbuf.size = bo_contig_space(h2c->mbuf);
 		outbuf.len = 0;