crypto: Define MODP_CUSTOM outside of IKE DH range

Before this fix it was possible to crash charon with an IKE_SA_INIT
message containing a KE payload with DH group MODP_CUSTOM(1025).
Defining MODP_CUSTOM outside of the two byte IKE DH identifier range
prevents it from getting negotiated.

Fixes CVE-2014-9221.

src/libstrongswan/plugins/openssl/openssl_diffie_hellman.c

@@ -38,7 +38,7 @@ struct private_openssl_diffie_hellman_t {
 	/**
 	 * Diffie Hellman group number.
 	 */
-	u_int16_t group;
+	diffie_hellman_group_t group;
 
 	/**
 	 * Diffie Hellman object

src/libstrongswan/plugins/openssl/openssl_ec_diffie_hellman.c

@@ -40,7 +40,7 @@ struct private_openssl_ec_diffie_hellman_t {
 	/**
 	 * Diffie Hellman group number.
 	 */
-	u_int16_t group;
+	diffie_hellman_group_t group;
 
 	/**
 	 * EC private (public) key