Eeschema, spice sim: fix a crash when no value is available.

onSimFinished() try to analyse the list of plots, but did not test
if there is available data before using it.
Fixes #8169
https://gitlab.com/kicad/code/kicad/issues/8169

eeschema/sim/ngspice.cpp

@@ -166,7 +166,7 @@ vector<double> NGSPICE::GetRealPlot( const string& aName, int aMaxLen )
         {
             for( int i = 0; i < length; i++ )
             {
-                assert( vi->v_compdata[i].cx_imag == 0.0 );
+                wxASSERT( vi->v_compdata[i].cx_imag == 0.0 );
                 data.push_back( vi->v_compdata[i].cx_real );
             }
         }

eeschema/sim/sim_plot_frame.cpp

@@ -1539,8 +1539,12 @@ void SIM_PLOT_FRAME::onSimFinished( wxCommandEvent& aEvent )
 
         for( const auto& vec : m_simulator->AllPlots() )
         {
-            double val = m_simulator->GetRealPlot( vec, 1 ).at( 0 );
+            std::vector<double> val_list = m_simulator->GetRealPlot( vec, 1 );
 
+            if( val_list.size() == 0 )      // The list of values can be empty!
+                continue;
+
+            double val = val_list.at( 0 );
             wxString      outLine, signal;
             SIM_PLOT_TYPE type = m_exporter->VectorToSignal( vec, signal );