MINOR: buffer: Use b_add()/bo_add() instead of accessing b->i/b->o.

Use the newly available functions instead of using the buffer fields directly.
diff --git a/src/buffer.c b/src/buffer.c
index fe756f7..b4b6e1a 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -101,7 +101,7 @@
 	if (len)
 		memcpy(pos, str, len);
 
-	b->i += delta;
+	b_add(b, delta);
 	b_realign_if_empty(b);
 
 	return delta;
@@ -141,7 +141,7 @@
 		pos[len + 1] = '\n';
 	}
 
-	b->i += delta;
+	b_add(b, delta);
 	return delta;
 }