doc/man3/OPENSSL_malloc.pod: explicitly document freeptr value on failures

doc/man3/OPENSSL_malloc.pod: explicitly document freeptr value on failures

Explicitly document that it is set to NULL, so can be passed to free()
without additional checks.

Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28059)