If we're part way through processing a record, or the application has
not released all the records then we should not free our buffer because
they are still needed.

CVE-2024-4741

In order to ensure we do not have a UAF we reset the rlayer.packet pointer
to NULL after we free it.

CVE-2024-4741

Test that attempting to free the buffers at points where they should not
be freed works as expected.

Follow on from CVE-2024-4741

The sslapitest has a helper function to load the dasync engine which is
useful for testing pipelining. We would like to have the same facility
from sslbuffertest, so we move the function to the common location
ssltestlib.c

Follow on from CVE-2024-4741

We extend the testing to test what happens when pipelining is in use.

Follow on from CVE-2024-4741

TLS pipelining provides the ability for libssl to read or write multiple
records in parallel. It requires special ciphers to do this, and there are
currently no built-in ciphers that provide this capability. However, the
dasync engine does have such a cipher, so we add a test for this capability
using that engine.

…lled

The pipeline input/output buf arrays must remain accessible to the
EVP_CIPHER_CTX until EVP_Cipher is subsequently called. This fixes an
asan error discovered by the newly added pipeline test.

During read pipelining we must ensure that the buffer is sufficiently large
to read enough data to fill our pipelines. We also remove some code that
moved data to the start of the packet if we can. This was unnecessary
because of later code which would end up moving it anyway. The earlier move
was also incorrect in the case that |clearold| was 0. This would cause the
read pipelining code to fail with sufficiently large records.

We shouldn't be putting more data into a pipeline than the value of
split_send_fragment.