Eeschema, DIALOG_PLOT_SCHEMATIC: fix incorrect unit of default pen width.

The default line width used to set the  default pen width is given in mils,
and the conversion to iu was missing.
Fix also a minor Coverity warning.

Fixes #4594
https://gitlab.com/kicad/code/kicad/issues/4594

eeschema/dialogs/dialog_plot_schematic.cpp

@@ -119,7 +119,8 @@ void DIALOG_PLOT_SCHEMATIC::initDlg()
 
         // Set the default line width (pen width which should be used for
         // items that do not have a pen size defined (like frame ref)
-        m_defaultLineWidth.SetValue( cfg->m_Drawing.default_line_thickness );
+        // the default line width is stored in mils in config
+        m_defaultLineWidth.SetValue( Mils2iu( cfg->m_Drawing.default_line_thickness ) );
     }
 
     // Initialize HPGL specific widgets