Fix "make test" seg fault with SCTP enabled

When config'd with "sctp" running "make test" causes a seg fault. This is
actually due to the way ssltest works - it dives under the covers and frees
up BIOs manually and so some BIOs are NULL when the SCTP code does not
expect it. The simplest fix is just to add some sanity checks to make sure
the BIOs aren't NULL before we use them.

This problem occurs in master and 1.0.2. The fix has also been applied to
1.0.1 to keep the code in sync.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(cherry picked from commit f75d5171)