MINOR: buffer: remove bo_ptr()

It was replaced by co_head() when a channel was known, otherwise b_head().
diff --git a/src/flt_http_comp.c b/src/flt_http_comp.c
index 2084397..d80dc08 100644
--- a/src/flt_http_comp.c
+++ b/src/flt_http_comp.c
@@ -721,7 +721,7 @@
 	/* Copy previous data from ib->o into ob->o */
 	if (ib->o > 0) {
 		left = bo_contig_data(ib);
-		memcpy(ob->p - ob->o, bo_ptr(ib), left);
+		memcpy(ob->p - ob->o, b_head(ib), left);
 		if (ib->o - left) /* second part of the buffer */
 			memcpy(ob->p - ob->o + left, ib->data, ib->o - left);
 	}