Fix for the CVE-2006-2940 fix

The newly introduced limit on DH modulus size could lead to a crash when
exerted.
This was fixed after the 0.9.8d release in the OpenSSL CVS:
http://cvs.openssl.org/chngview?cn=15607

--- crypto/dh/dh_key.c	2005-08-20 20:35:53.000000000 +0200
+++ crypto/dh/dh_key.c	2006-10-04 13:25:02.000000000 +0200
@@ -173,7 +173,7 @@
 
 static int compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)
 	{
-	BN_CTX *ctx;
+	BN_CTX *ctx=NULL;
 	BN_MONT_CTX *mont=NULL;
 	BIGNUM *tmp;
 	int ret= -1;