systemc: Don't schedule the update phase if the event queue is null.

systemc: Don't schedule the update phase if the event queue is null.

That happens when an update is requested before the initialization
phase has started. In that case, the update phase will be manually run
and no event needs to be scheduled, even if that was possible.

Change-Id: I2008e29064d282f82bd1935dbe5b94407aa925b0
Reviewed-on: https://gem5-review.googlesource.com/12078
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>