Some small fixes to tst-pipe.cc

tst-pipe.cc read a buffer after freeing it, which could have theoretically
caused segfaults (it didn't in practice, but better fix this oversight).

Also, it forgot to return a return code, so it doesn't play nicely in
a test framework like testrunner.so. I'm surprised the C++ compiler wasn't
bothered by an int main() not returning an int.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>