ASoC: SOF: fix fault at driver unload after failed probe

If sof_machine_check() fails during driver probe, the IPC
state is not initialized and this will lead to a NULL
dereference at driver unload. Example log is as follows:

[ 1535.980630] sof-audio-pci 0000:00:1f.3: error: no matching ASoC machine driver found - aborting probe
[ 1535.980631] sof-audio-pci 0000:00:1f.3: error: failed to get machine info -19
[ 1535.980632] sof-audio-pci 0000:00:1f.3: error: sof_probe_work failed err: -19
[ 1550.798373] BUG: kernel NULL pointer dereference, address: 0000000000000008
...
[ 1550.798393] Call Trace:
[ 1550.798397]  snd_sof_ipc_free+0x15/0x30 [snd_sof]
[ 1550.798399]  snd_sof_device_remove+0x29/0xa0 [snd_sof]
[ 1550.798400]  sof_pci_remove+0x10/0x30 [snd_sof_pci]

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20191218000518.5830-2-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/ipc.c

@@ -826,6 +826,9 @@ void snd_sof_ipc_free(struct snd_sof_dev *sdev)
 {
 	struct snd_sof_ipc *ipc = sdev->ipc;
 
+	if (!ipc)
+		return;
+
 	/* disable sending of ipc's */
 	mutex_lock(&ipc->tx_mutex);
 	ipc->disable_ipc_tx = true;