Fix a bug in fixed_sized_callback.

This is uninitializing the ring buffer before the device which results
in a situation where a stray data callback may try touching the buffer
after it's be uninitialized.

examples/fixed_size_callback.c

@@ -132,8 +132,8 @@ int main(int argc, char** argv)
     printf("Press Enter to quit...\n");
     getchar();
 
-    ma_pcm_rb_uninit(&g_rb);
     ma_device_uninit(&device);
+    ma_pcm_rb_uninit(&g_rb);
 
     (void)argc;
     (void)argv;