Add missing OFB entry to null ciphersuite

The OFB entry has been omitted from the the null cipher suite definition,
null_base_info.

library/cipher_wrap.c

diff --git a/library/cipher_wrap.c b/library/cipher_wrap.c
index ef47037..33c71f1 100644
--- a/library/cipher_wrap.c
+++ b/library/cipher_wrap.c
@@ -1198,6 +1198,9 @@
 #if defined(MBEDTLS_CIPHER_MODE_CFB)
     NULL,
 #endif
+#if defined(MBEDTLS_CIPHER_MODE_OFB)
+    NULL,
+#endif
 #if defined(MBEDTLS_CIPHER_MODE_CTR)
     NULL,
 #endif