ssl_tls13_parse_client_hello() does not handle legacy_compression_methods properly

[waleed-elmelegy-arm]

Summary

ssl_tls13_parse_client_hello() assumes legacy_compression_methods length will always be zero which is true for TLS 1.3 However if TLS 1.3 is enabled by default all clientHello requests (including TLS 1.2 requests) will go to ssl_tls13_parse_client_hello() first and then passed to TLS 1.2 parsing function later but processing legacy_compression_methods happens earlier so it might be not zero if it's a TLS 1.2 request.

System information

Mbed TLS version (number or commit id): 3.6.0
Operating system and version: all
Configuration (if not default, please attach mbedtls_config.h): defualt
Compiler and options (if you used a pre-built binary, please indicate how you obtained it): N/A
Additional environment information:

Expected behavior

ssl_tls13_parse_client_hello() returns SSL_CLIENT_HELLO_TLS1_2 if legacy_compression_methods wasn't zero

Actual behavior

ssl_tls13_parse_client_hello() continues normally but parsing of subsequent data is wrong.

Steps to reproduce

send TLS 1.2 ClientHello request with non zero legacy_compression_methods

Additional information