SDL: netbsdaudio: Minor fix.

https://github.com/libsdl-org/SDL/commit/9fa4a6ef8789ead45a670bce1910748e5d145b58

From 9fa4a6ef8789ead45a670bce1910748e5d145b58 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Fri, 21 Jul 2023 10:56:15 -0400
Subject: [PATCH] netbsdaudio: Minor fix.

---
 src/audio/netbsd/SDL_netbsdaudio.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/audio/netbsd/SDL_netbsdaudio.c b/src/audio/netbsd/SDL_netbsdaudio.c
index 86ebd09b0a65..4165b056df51 100644
--- a/src/audio/netbsd/SDL_netbsdaudio.c
+++ b/src/audio/netbsd/SDL_netbsdaudio.c
@@ -204,6 +204,7 @@ static void NETBSDAUDIO_CloseDevice(SDL_AudioDevice *device)
         }
         SDL_free(device->hidden->mixbuf);
         SDL_free(device->hidden);
+        device->hidden = NULL;
     }
 }