Check whether buffers have actually been allocated/freed

Check whether buffers have actually been allocated/freed

In the sslbuffertest we test the operation of SSL_alloc_buffers() and
SSL_free_buffers(). However this was done entirely using the public API,
and did not confirm that the buffers were actually allocated/freed. We
now extend the test to confirm this.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19472)