treebuilder: exit early if running OOM in `write_with_buffer`

treebuilder: exit early if running OOM in `write_with_buffer`

While writing the tree inside of a buffer, we check whether the buffer
runs out of memory after each tree entry. While we set the error code as
soon as we detect the OOM situation, we happily proceed iterating over
the entries. This is not useful at all, as we will try to write into the
buffer repeatedly, which cannot work.

Fix this by exiting as soon as we are OOM.