#3 Silence valgrind memcheck error by memset()-ing pkt to 0

[Paul Wilkinson]

valgrind reports:

Syscall param write(buf) points to uninitialised byte(s)

In this case, it is a false positive, but adding this single memset at
attach start-up means the program is now clean when run with "valgrind
--trace-children=yes dtach -c ...", which can make future debugging
easier.