Fix failure to clear list before reusing it.

Fixes https://gitlab.com/kicad/code/kicad/issues/6927

eeschema/dialogs/dialog_change_symbols.cpp

@@ -272,6 +272,7 @@ void DIALOG_CHANGE_SYMBOLS::updateFieldsList()
             if( !isMatch( symbol, &instance ) )
                 continue;
 
+            fields.clear();
             symbol->GetFields( fields, false );
 
             for( unsigned i = MANDATORY_FIELDS; i < fields.size(); ++i )