audio: Fix warning C4701: potentially uninitialized local variable 'isstack' used

This warning is actually wrong, as the variable is only used if the string pointer is valid, in which case it was initialized, but this is needed to quiet MSVC.

src/audio/SDL_audio.c

@@ -1502,7 +1502,7 @@ SDL_AudioDevice *SDL_FindPhysicalAudioDeviceByHandle(void *handle)
 
 const char *SDL_GetAudioDeviceName(SDL_AudioDeviceID devid)
 {
-    bool isstack;
+    bool isstack = false;
     char *string = NULL;
     const char *result = NULL;
     SDL_AudioDevice *device = NULL;