Fix ex_data locks issue

Travis identified a problem with freeing the ex_data locks which wasn't
quite right in ff234405

. Trying to fix it identified a further problem:
the ex_data locks are cleaned up by OPENSSL_cleanup(), which is called
explicitly by CRYPTO_mem_leaks(), but then later the BIO passed to
CRYPTO_mem_leaks() is freed. An attempt is then made to use the ex_data
lock already freed.

Reviewed-by: Tim Hudson <tjh@openssl.org>