MINOR: buffer: make bo_putchar() use b_tail()

It's possible because we can't call bo_putchar() with i != 0.
diff --git a/include/common/buffer.h b/include/common/buffer.h
index a05d58d..976ea02 100644
--- a/include/common/buffer.h
+++ b/include/common/buffer.h
@@ -205,7 +205,7 @@
 {
 	if (b_data(b) == b->size)
 		return;
-	*b->p = c;
+	*b_tail(b) = c;
 	b->p = b_peek(b, b->o + 1);
 	b->o++;
 }