dsp: Refuse to initialize if there aren't any Open Sound System devices.

This prevents the dsp target from stealing the audio subsystem but not
being able to produce sound, so other audio targets further down the list
can make an attempt instead.

Thanks to Frank Praznik who did a lot of the research on this problem!

src/audio/dsp/SDL_dspaudio.c

@@ -292,9 +292,24 @@ DSP_FlushCapture(_THIS)
     }
 }
 
+static SDL_bool InitTimeDevicesExist = SDL_FALSE;
+static int
+look_for_devices_test(int fd)
+{
+    InitTimeDevicesExist = SDL_TRUE;  /* note that _something_ exists. */
+    /* Don't add to the device list, we're just seeing if any devices exist. */
+    return 0;
+}
+
 static int
 DSP_Init(SDL_AudioDriverImpl * impl)
 {
+    InitTimeDevicesExist = SDL_FALSE;
+    SDL_EnumUnixAudioDevices(0, look_for_devices_test);
+    if (!InitTimeDevicesExist) {
+        return 0;  /* maybe try a different backend. */
+    }
+
     /* Set the function pointers */
     impl->DetectDevices = DSP_DetectDevices;
     impl->OpenDevice = DSP_OpenDevice;