sim: Fix cosimulation with nested modules having unconnected inputs

When assigning values to input ports of nested modules in cosimulation,
sim needs to find the actual driver of the signal to perform the
assignment. The existing code didn't handle unconnected inputs in that
scenario.

passes/sat/sim.cc

@@ -223,6 +223,7 @@ struct SimInstance
 
 			if (wire->port_input && instance != nullptr && parent != nullptr) {
 				for (int i = 0; i < GetSize(sig); i++) {
+					if (instance->hasPort(wire->name))
 						in_parent_drivers.emplace(sig[i], parent->sigmap(instance->getPort(wire->name)[i]));
 				}
 			}