fix: support pid > 65535

[apopov-app]

Problem

MTProxy crashed due to assert (!(p & 0xffff0000)) in init_common_PID: the code assumed PID always fits in 16 bits. On systems with pid_max > 65535 (e.g. pid 94936, 95241), the assert triggered
and the proxy crashed.

Solution (workaround)

Removed the assert and store only the lower 16 bits of the PID instead of the full value:

PID.pid = (unsigned short) (p & 0xffff);

Why not 32 bits

This is a workaround. Ideally we would pass a 32-bit pid, but that is not possible:

• process_id is serialized in the RPC handshake between MTProxy and Telegram DC servers
• The packet format is fixed: 32 bytes, pid is 16 bits
• We cannot change Telegram servers or the protocol
• With a 32-bit pid, the handshake size grows from 32 to 40 bytes — DC expects 32, so the protocol breaks
  So we only have truncation: instead of a crash, there may be pid collisions when pid > 65535 (rarely critical for MTProxy).