Add fetch_hand_message in generic

This function is one common function in generic file, get it from
the encrypted extension and submit one patch independently.

Signed-off-by: XiaokangQian <xiaokang.qian@arm.com>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index 5c20f29..4aaafa5 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -28,6 +28,38 @@
 
 #include "ssl_misc.h"
 
+int mbedtls_ssl_tls1_3_fetch_handshake_msg( mbedtls_ssl_context *ssl,
+                                     unsigned hs_type,
+                                     unsigned char **buf,
+                                     size_t *buflen )
+{
+    int ret;
+
+    if( ( ret = mbedtls_ssl_read_record( ssl, 0 ) ) != 0 )
+    {
+        MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_read_record", ret );
+        goto cleanup;
+    }
+
+    if( ssl->in_msgtype != MBEDTLS_SSL_MSG_HANDSHAKE         ||
+        ssl->in_msg[0]  != hs_type )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad certificate verify message" ) );
+        MBEDTLS_SSL_PEND_FATAL_ALERT( MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE,
+                              MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE );
+        ret = MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE;
+        goto cleanup;
+    }
+
+    *buf    = ssl->in_msg   + 4;
+    *buflen = ssl->in_hslen - 4;
+
+
+cleanup:
+
+    return( ret );
+}
+
 int mbedtls_ssl_tls13_start_handshake_msg( mbedtls_ssl_context *ssl,
                                            unsigned hs_type,
                                            unsigned char **buf,