Disk audio target was using this->hidden->mixlen before we set it.

src/audio/disk/SDL_diskaudio.c

@@ -114,6 +114,10 @@ DISKAUD_OpenDevice(_THIS, const char *devname, int iscapture)
     }
     SDL_memset(this->hidden, 0, sizeof(*this->hidden));
 
+    this->hidden->mixlen = this->spec.size;
+    this->hidden->write_delay =
+        (envr) ? SDL_atoi(envr) : DISKDEFAULT_WRITEDELAY;
+
     /* Open the audio device */
     this->hidden->output = SDL_RWFromFile(fname, "wb");
     if (this->hidden->output == NULL) {
@@ -129,10 +133,6 @@ DISKAUD_OpenDevice(_THIS, const char *devname, int iscapture)
     }
     SDL_memset(this->hidden->mixbuf, this->spec.silence, this->spec.size);
 
-    this->hidden->mixlen = this->spec.size;
-    this->hidden->write_delay =
-        (envr) ? SDL_atoi(envr) : DISKDEFAULT_WRITEDELAY;
-
 #if HAVE_STDIO_H
     fprintf(stderr,
             "WARNING: You are using the SDL disk writer audio driver!\n"