Fix packet filename in "Gruned message" log entry on Win32

e.g. "Grunged message (type 2) from 4:80/1 at offset 58 in packet: (null)"

off_t is 64-bit, but long in 32-bit (on Win32), so needs a typecast here in
lprintf() call (otherwise, the upper 4 bytes of the offset are interpreted
as the string address for the corresponding '%s' argument, the filename).