SDL: audio: Fixed assertion failure if trying to use dummy backend.

SDL Commits
1 
From 0f4aba7bcdc6aa8671c1f93700266b0e8907cc70 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 6 Apr 2021 18:34:53 -0400
Subject: [PATCH] audio: Fixed assertion failure if trying to use dummy
 backend.

---
 src/audio/dummy/SDL_dummyaudio.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/audio/dummy/SDL_dummyaudio.c b/src/audio/dummy/SDL_dummyaudio.c
index 6d6d61c0a..acd54b89a 100644
--- a/src/audio/dummy/SDL_dummyaudio.c
+++ b/src/audio/dummy/SDL_dummyaudio.c
@@ -30,6 +30,7 @@
 static int
 DUMMYAUDIO_OpenDevice(_THIS, void *handle, const char *devname, int iscapture)
 {
+    _this->hidden = (void *) 0x1;  /* just something non-NULL */
     return 0;                   /* always succeeds. */
 }