Remove directly access for tls13_sig_algs

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index c789ed4..9ffd078 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -179,7 +179,7 @@
      * Write supported_signature_algorithms
      */
     supported_sig_alg = p;
-    for( const uint16_t *sig_alg = ssl->conf->tls13_sig_algs;
+    for( const uint16_t *sig_alg = mbedtls_ssl_conf_get_sig_algs( ssl->conf );
          *sig_alg != MBEDTLS_TLS1_3_SIG_NONE; sig_alg++ )
     {
         MBEDTLS_SSL_CHK_BUF_PTR( p, end, 2 );
@@ -284,13 +284,12 @@
 }
 
 static int ssl_tls13_sig_alg_is_offered( const mbedtls_ssl_context *ssl,
-                                         uint16_t sig_alg )
+                                         uint16_t received_sig_alg )
 {
-    const uint16_t *tls13_sig_alg = ssl->conf->tls13_sig_algs;
-
-    for( ; *tls13_sig_alg != MBEDTLS_TLS1_3_SIG_NONE ; tls13_sig_alg++ )
+    for( const uint16_t *sig_alg = mbedtls_ssl_conf_get_sig_algs( ssl->conf );
+         *sig_alg != MBEDTLS_TLS1_3_SIG_NONE; sig_alg++ )
     {
-        if( *tls13_sig_alg == sig_alg )
+        if( *sig_alg == received_sig_alg )
             return( 1 );
     }
     return( 0 );