fix Coverity 1604662

fix Coverity 1604662

Coverity flagged an issue in our bio_enc tests in which we failed to
check the return code of BIO_read for an error condition which can lead
to our length computation going backwards.

Just check the error code before adding it to length

Fixes openssl/project#779

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/25006)