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/library/ssl_cli.c b/library/ssl_cli.c
index 2c209d3..d8b1ce0 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -4254,6 +4254,7 @@
            mbedtls_ssl_handshake_wrapup( ssl );
            break;
 
+       case MBEDTLS_SSL_INVALID:
        default:
            MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid state %d", ssl->state ) );
            return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );