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_srv.c b/library/ssl_srv.c
index fb64a2b..1a341c4 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -4818,6 +4818,7 @@
             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 );