The calculation didn't take into account the NULL terminator. That lead
to overwriting the palloc'd buffer by one byte, if the input consists
entirely of backslashes. For example "format('%L', E'\\')".

Fixes bug #14468. Backpatch to all supported versions.

Report: https://www.postgresql.org/message-id/20161216105001.13334.42819%40wrigleys.postgresql.org