Initialise ctx to NULL to avoid uninitialized free, noticed by

Steve Kiernan

crypto/dh/dh_key.c

@@ -175,7 +175,7 @@ err:
 
 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;