Prevent a crash if command 'save n1' has been given on a digital node n1.

Improved error message.

src/frontend/outitf.c

@@ -398,6 +398,7 @@ beginPlot(JOB *analysisPtr, CKTcircuit *circuitPtr, char *cktName, char *analNam
         {
             fprintf(cp_err, "Error: no data saved for %s; analysis not run\n",
                     spice_analysis_get_description(analysisPtr->JOBtype));
+            fprintf(cp_err, "    Command 'save' on a digital node?\n");
             return E_NOTFOUND;
         }
 

src/xspice/evt/evtprint.c

@@ -625,6 +625,9 @@ EVTprintvcd(wordlist *wl)
     more = MIF_FALSE;
     next_step = 1e30;
     for (i = 0; i < nargs; i++) {
+        /* prevent crash if save(d1), and d1 is a digital node */
+        if (node_data[i] == NULL)
+            continue;
         step = node_data[i]->step;
         g_evt_udn_info[udn_index[i]]->print_val
             (node_data[i]->node_value, "all", &value);