serial port comm: prevent read in freed memory

In channels/serial/client/serial_main.c::__serial_check_fds the calls to serial_process_irp_read, serial_process_irp_write, and irp->Complete free the memory pointed to by 'irp'

This invalidates the call to 'prev->IoStatus' later in the function.

Keeping track for 'completion' to prevent a read in an already free'd memory block.