tls: Align set and usage check for PSK

Check that the identity length is not
zero in ssl_conf_set_psk_identity()
as it is done in
mbedtls_ssl_conf_has_static_psk().

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

library/ssl_tls.c

@@ -1654,6 +1654,7 @@ static int ssl_conf_set_psk_identity( mbedtls_ssl_config *conf,
 {
     /* Identity len will be encoded on two bytes */
     if( psk_identity               == NULL ||
+        psk_identity_len           == 0    ||
         ( psk_identity_len >> 16 ) != 0    ||
         psk_identity_len > MBEDTLS_SSL_OUT_CONTENT_LEN )
     {