pgsql: Fix truncation rules for base64 encoding

Fix truncation rules for base64 encoding

Commit e1d917182 added support for base64url encoding, a base64 variant
intended to be safe for usage in URLs and filenames. The padding rules
for base64url and base64 differ in that base64url require no extra '='
padding, but the commit unintentionally relaxed this requirement for
base64 as well. Fix by making sure that the truncation logic check
for the encoding and add a test to make sure.

Backpatch down to v19 where support for base64url was introduced.

Author: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: David E. Wheeler <david(at)justatheory(dot)com>
Discussion: https://postgr.es/m/3258FC72-F5E1-40B9-B5D7-64478CAF7728@yesql.se
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5f57f58179bf48805558ac0494851e93fe0eca76

Modified Files
--------------
src/backend/utils/adt/encode.c | 4 ++--
src/test/regress/expected/strings.out | 4 ++++
src/test/regress/sql/strings.sql | 3 +++
3 files changed, 9 insertions(+), 2 deletions(-)