Use correct value when calculating audio conversion length.

  Fixes Bugzilla #477.

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%403839

src/audio/SDL_audio.c

@@ -532,7 +532,7 @@ int SDL_OpenAudio(SDL_AudioSpec *desired, SDL_AudioSpec *obtained)
 			return(-1);
 		}
 		if ( audio->convert.needed ) {
-			audio->convert.len = (int) ( ((double) desired->size) /
+			audio->convert.len = (int) ( ((double) audio->spec.size) /
                                           audio->convert.len_ratio );
 			audio->convert.buf =(Uint8 *)SDL_AllocAudioMem(
 			   audio->convert.len*audio->convert.len_mult);