Fix for sound loading even when sound is off

[ceeac]

When starting Simutrans with sound turned off (using the -nosound command line flag), sound_desc_t::get_sound_id tries to load the sound anyway. This patch fixes the bug.

[prissi]

It was allowed to turn on sound via the dialog panel, even if started with -nosound. (same for nomidi). That was a change 8 years ago.

[Matthew]

As a player, I am surprised to read that. I do not want Simutrans to load sound files when the -nosound parameter is used. When the pakset and/or savegame are so large that I am running low on RAM, it is very useful to be able to skip loading sound files.

[prissi]

You can rename the sound folder ... But the sound fiels are tiny compared to the images in memory.