Use a flag in SSL3_BUFFER to track when an application buffer is reused.

Use a flag in SSL3_BUFFER to track when an application buffer is reused.

With KTLS, writes to an SSL connection store the application buffer
pointer directly in the 'buf' member instead of allocating a separate
buffer to hold the encrypted data.  As a result,
ssl3_release_write_buffer() has to avoid freeing these 'buf' pointers.

Previously, ssl3_release_write_buffer() checked for KTLS being enabled
on the write BIO to determine if a buffer should be freed.  However, a
buffer can outlive a BIO.  For example, 'openssl s_time' creates new
write BIOs when reusing sessions.  Since the new BIO did not have KTLS
enabled at the start of a connection, ssl3_release_write_buffer()
would incorrectly try to free the 'buf' pointer from the previous KTLS
connection.  To fix, track the state of 'buf' explicitly in
SSL3_BUFFER to determine if the 'buf' should be freed or simply
cleared.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10489)