When running a command in the background, do_main_thread() passes the
command line in a std::vector pointer to a new pthread. Unfortunately,
soon afterwards the vector can go out of scope and the result is a
crash. Fix this oversight.

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