Encoder : Fix floating pointer when OSSL_ENCODER_to_data() is called

Encoder : Fix floating pointer when OSSL_ENCODER_to_data() is called
twice.

Fixes #26862

This only happens when using the FIPS provider, since it needs to export
the key.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26891)

(cherry picked from commit c2f4d7aae1c7c726eb1f8226d3d454dfd9754758)