main: fix heap overflow in dbus-launch wrapping

I have discovered a heap overflow with the help of an address sanitizer.

The require_dbus_session() function has this code:

        new_argv = g_malloc (argc + 3 * sizeof (*argv));

The intention is to allocate space for (argc + 3) pointers. However obviously a
parenthesis is missing, therefore only argc bytes + 3 * pointer size gets
allocated, which is insufficient space. This leads to invalid memory writes.

The fix is trivial: Parentheses around argc + 3.

https://bugzilla.gnome.org/show_bug.cgi?id=768441