connection, transport: Assert that timeout >= -1 where it matters

connection, transport: Assert that timeout >= -1 where it matters

We recommend that assertions are disabled in production builds of dbus,
which means that they are "cheap" to add to development builds as
"executable documentation" for our assumptions.

Lower-level code assumes that timeouts must be either -1 to block
forever, or non-negative to block for a finite time (but possibly 0,
to poll without blocking).

Signed-off-by: Simon McVittie <smcv@collabora.com>