com_measure2.c, write a fft vector (sp plot) with complex data

and real frequency to file and load it again, frequency now has become
a complex number as well. This was not recognized, thus ngspice crashed
(see https://sourceforge.net/p/ngspice/discussion/127605/thread/8cccd69c/
by Michal Janik - 2014-11-04

src/frontend/com_measure2.c

@@ -758,7 +758,11 @@ measure_minMaxAvg(
                 value = get_value(meas, d, i); //d->v_compdata[i].cx_real;
             else
                 value = d->v_realdata[i];
-            svalue = dScale->v_realdata[i];
+            if (dScale->v_realdata)
+                svalue = dScale->v_realdata[i];
+            else
+                /* may happen if you write an sp vector and load it again */
+                svalue = dScale->v_compdata[i].cx_real;
         } else {
             value = d->v_realdata[i];
             svalue = dScale->v_realdata[i];