Ignore TS_MULTIFRAGMENTUPDATE_CAPABILITYSET from client if fp disabled

[matt335672]

Fixes #1569

rdesktop 1.9.0 (as delivered with Ubuntu 20.04) sends a TS_MULTIFRAGMENTUPDATE_CAPABILITYSET structure ([MS-RDPBCGR] 2.2.7.2.6) with a MaxRequestSize of 65535.

If the user has disabled fastpath output using use_fastpath in xrdp.ini, the following happens:-

• An output buffer is created of size 32768 in xrdp_orders_create().
• Receipt of TS_MULTIFRAGMENTUPDATE_CAPABILITYSET in xrdp_caps_process_multifragmentupdate() sets client_info.max_fastpath_frag_bytes to 65535.
• This value is used by xrdp_orders_check() va the MAX_ORDERS_SIZE macro to determine when to send the orders buffer. Consequently the buffer overflows when being written to.

This PR simply ignores any sent MaxRequestSize value if fastpath output is disabled.

[metalefty]

ref. 2.2.7.2.6 Multifragment Update Capability Set (TS_MULTIFRAGMENTUPDATE_CAPABILITYSET)

[metalefty]

Fair enough. Nice work.

[metalefty]

Merge it.