channel_binding's longest allowed value is not "7", it is actually "8".
gssencmode also got that wrong.

A similar mistake has been fixed as of f4051e36.

Backpatch down to v12, where gssencmode has been introduced.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20200128053633.GD1552@paquier.xyz
Backpatch-through: 12