sim: Fix -Werror=maybe-uninitialized in system.cc

sim: Fix -Werror=maybe-uninitialized in system.cc

The patch is simply initializing when to 0 before unserializing
the real value

Change-Id: I4e19eeafa9334116b440948af1943f3835803671
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30594
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>