ALSA: pcmtest: fix reference leak on failed device registration

When platform_device_register() fails in mod_init(), the embedded struct
device in pcmtst_pdev has already been initialized by
device_initialize(), but the failure path returns the error without
dropping the device reference for the current platform device:

  mod_init()
    -> platform_device_register(&pcmtst_pdev)
       -> device_initialize(&pcmtst_pdev.dev)
       -> setup_pdev_dma_masks(&pcmtst_pdev)
       -> platform_device_add(&pcmtst_pdev)

This leads to a reference leak when platform_device_register() fails.
Fix this by calling platform_device_put() before returning the error.

The issue was identified by a static analysis tool I developed and
confirmed by manual review.

Fixes: 315a3d57c6 ("ALSA: Implement the new Virtual PCM Test Driver")
Cc: [email protected]
Signed-off-by: Guangshuo Li <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

sound/drivers/pcmtest.c

@@ -756,8 +756,10 @@ static int __init mod_init(void)
 	if (err)
 		return err;
 	err = platform_device_register(&pcmtst_pdev);
-	if (err)
+	if (err) {
+		platform_device_put(&pcmtst_pdev);
 		return err;
+	}
 	err = platform_driver_register(&pcmtst_pdrv);
 	if (err)
 		platform_device_unregister(&pcmtst_pdev);