Fix segfault when auto converting stream #151991

--- src/audio/SDL_audio.c
+++ src/audio/SDL_audio.c
@@ -591,7 +591,7 @@
 			return(-1);
 		}
 		if ( audio->convert.needed ) {
-			audio->convert.len = desired->size;
+			audio->convert.len = desired->size / audio->convert.len_ratio;
 			audio->convert.buf =(Uint8 *)SDL_AllocAudioMem(
 			   audio->convert.len*audio->convert.len_mult);
 			if ( audio->convert.buf == NULL ) {