Fix overflow in EVP_EncodeFinal

Fix overflow in EVP_EncodeFinal

https://scan5.scan.coverity.com/#/project-view/60762/10222?selectedIssue=1677829

With recent changes, evp_encodeblock_int may return a negative value,
which EVP_EncodeFinal does not anticipate.  As the latter sets out[ret]
to "\0" where ret is the return value of evp_encodeblock_int, we may
underflow the array index and access invalid memory locations.

Only update the output buffer if the return value is greater or equal to
zero.

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Nikola Pajkovsky <nikolap@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29525)