Fix sdf_interconnect4 failure and reenable

ivtest/vvp_tests/sdf_interconnect4.json

@@ -1,5 +1,5 @@
 {
-    "type"          : "EF",
+    "type"          : "normal",
     "source"        : "sdf_interconnect4.v",
     "iverilog-args" : [ "-Ttyp", "-ginterconnect", "-gspecify" ],
     "gold"   : "sdf_interconnect4",

vvp/vpi_priv.cc

@@ -1818,7 +1818,9 @@ vpiHandle vpi_handle_multi(PLI_INT32 type,
 			      prev = vvp_net_ptr_t(nullptr, 0);
 			} else {
 			      prev = previous_node;
-			      cur = prev.ptr()->port[prev.port()];
+			      if (prev.ptr()) {
+				    cur = prev.ptr()->port[prev.port()];
+			      }
 			}
 		  }