Don't raise an error on retryable read in a BIO_s_dgram_pair()

Don't raise an error on retryable read in a BIO_s_dgram_pair()

This results in spurious errors appearing on the queue in normal
operation, e.g. calling SSL_tick() with a QUIC connection will succeed,
but an error will end up on the queue anyway.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20030)