Add enumeration for invalid state

The invalid state can be used when state-mismatch is noticed.
The invalid state should report a FI-alert upwards.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 8008b51..f147069 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -583,6 +583,7 @@
     MBEDTLS_SSL_HANDSHAKE_OVER,
     MBEDTLS_SSL_SERVER_NEW_SESSION_TICKET,
     MBEDTLS_SSL_SERVER_HELLO_VERIFY_REQUEST_SENT,
+    MBEDTLS_SSL_INVALID
 }
 mbedtls_ssl_states;