Do not open the output file twice

7f229b4f caused the output file to get opened twice, while
some types of files (e.g. named pipes) can be only opened once.

After this change Session::applyCommandLine opens the output file
only when there is an error to print.

include/internal/catch_session.cpp

@@ -171,9 +171,9 @@ namespace Catch {
             return 1;
 
         auto result = m_cli.parse( clara::Args( argc, argv ) );
-        config();
-        getCurrentMutableContext().setConfig( m_config );
         if( !result ) {
+            config();
+            getCurrentMutableContext().setConfig(m_config);
             Catch::cerr()
                 << Colour( Colour::Red )
                 << "\nError(s) in input:\n"