mbedtls: Add bounds check before length read

This is part of the patch for CVE-2018-9989.
Cherry-picked from 740b218386
Ref. https://github.com/espressif/esp-idf/issues/1860

components/mbedtls/library/ssl_cli.c

@@ -2049,6 +2049,12 @@ static int ssl_parse_server_psk_hint( mbedtls_ssl_context *ssl,
      *
      * opaque psk_identity_hint<0..2^16-1>;
      */
+    if( (*p) > end - 2 )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad server key exchange message "
+                                    "(psk_identity_hint length)" ) );
+        return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
+    }
     len = (*p)[0] << 8 | (*p)[1];
     *p += 2;