Prior to this commit our `max_allowed_packed` would fail any time the
`len` of the data being written to the buffer was itself greater than
the configured `max_allowed_packet`.

This is because both the `len` and the `max_allowed_packet`
(aka `builder->packet_max_length`) are unsigned (`size_t`).

This commit bypasses the problem by adding to the left side instead of
subtracting from the right.