Don't call poll(2) with timeout < -1

On Linux, poll accepts any negative value as infinity.
On at least FreeBSD and NetBSD, only -1 is acceptable.

[adjusted whitespace for correct coding style -smcv]
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=78480
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

dbus/dbus-sysdeps-unix.c

@@ -2573,6 +2573,11 @@ _dbus_poll (DBusPollFD *fds,
       _DBUS_STRUCT_OFFSET (DBusPollFD, revents) ==
       _DBUS_STRUCT_OFFSET (struct pollfd, revents))
     {
+      if (timeout_milliseconds < -1)
+        {
+          timeout_milliseconds = -1;
+        }
+
       return poll ((struct pollfd*) fds,
                    n_fds,
                    timeout_milliseconds);