Fix magic + 20 in PEM_ASN1_write_bio

Fix magic + 20 in PEM_ASN1_write_bio

Fixes #26476

In the file crypto/pem/pem_lib.c the function had a +20 to account for
padding in the data size, however this was recognized to not be up to
standard quality. Instead it has now been updated to use the static
maximum block size and uses that for the calculation as opposed to a +20.

CLA: trivial

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26526)