This commit adds a secure heap facility to OpenSSL.  From the PODpage:

In order to help protect applications (particularly long-running servers)
from pointer overruns or underruns that could return arbitrary data from
the program's dynamic memory area, where keys and other sensitive
information might be stored, OpenSSL supports the concept of a "secure heap."

If a secure heap is used, then all B<BIGNUM> values are stored in that heap.
This is more than strictly necessary -- the public part of an RSA key, for
example, need not be kept private -- but it is a secure approach and least
invasive to the OpenSSL source code.