Add missing OFB entry to null ciphersuite

The OFB entry has been omitted from the the null cipher suite definition,
null_base_info.
diff --git a/library/cipher_wrap.c b/library/cipher_wrap.c
index f7d5737..901a2ca 100644
--- a/library/cipher_wrap.c
+++ b/library/cipher_wrap.c
@@ -1399,6 +1399,9 @@
 #if defined(MBEDTLS_CIPHER_MODE_CFB)
     NULL,
 #endif
+#if defined(MBEDTLS_CIPHER_MODE_OFB)
+    NULL,
+#endif
 #if defined(MBEDTLS_CIPHER_MODE_CTR)
     NULL,
 #endif