Fix small overestimation of base64 encoding output length.

Fix small overestimation of base64 encoding output length.

pg_base64_enc_len() and its clones overestimated the output
length by up to 2 bytes, as a result of sloppy thinking about
where to divide.  No callers require a precise estimate, so
this has no consequences worse than palloc'ing a byte or two
more than necessary.  We might as well get it right though.

This bug is very ancient, dating to commit 79d78bb26 which
added encode.c.  (The other instances were presumably copied
from there.)  Still, it doesn't quite seem worth back-patching.

Oleg Tselebrovskiy

Discussion: https://postgr.es/m/f94da55286a63022150bc266afdab754@postgrespro.ru