tranif control ports handle nil inputs

In certain special cases, it is possible for the tranif control input
to be nil. (probably only during startup.) Treat that case as HiZ.

vvp/island_tran.cc

@@ -114,7 +114,11 @@ bool vvp_island_branch_tran::run_test_enabled()
       }
 
       enabled_flag = false;
-      vvp_bit4_t enable_val = ep->invalue.value(0).value();
+      vvp_bit4_t enable_val;
+      if (ep->invalue.size() == 0)
+	    enable_val = BIT4_Z;
+      else
+	    enable_val = ep->invalue.value(0).value();
 
       if (active_high==true && enable_val != BIT4_1)
 	    return false;