Don't crash if we fail to find a serializer for the current provider
Don't crash if we fail to find a serializer for the current provider

The code was assuming that a serializer would always be found - but this
may not be the case.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11271)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl