dmaengine: dmatest: Check list for emptiness before access its last entry

After writing a garbage to the channel we get an Oops in dmatest_chan_set()
due to access to last entry in the empty list.

[  212.670672] BUG: unable to handle page fault for address: fffffff000000020
[  212.677562] #PF: supervisor read access in kernel mode
[  212.682702] #PF: error_code(0x0000) - not-present page
...
[  212.710074] RIP: 0010:dmatest_chan_set+0x149/0x2d0 [dmatest]
[  212.715739] Code: e8 cc f9 ff ff 48 8b 1d 0d 55 00 00 48 83 7b 10 00 0f 84 63 01 00 00 48 c7 c7 d0 65 4d c0 e8 ee 4a f5 e1 48 89 c6 48 8b 43 10 <48> 8b 40 20 48 8b 78 58 48 85 ff 0f 84 f5 00 00 00 e8 b1 41 f5 e1

Fix this by checking list for emptiness before accessing its last entry.

Fixes: d53513d5dc ("dmaengine: dmatest: Add support for multi channel testing")
Cc: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200922115847.30100-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/dmatest.c

@@ -1255,15 +1255,14 @@ static int dmatest_chan_set(const char *val, const struct kernel_param *kp)
 	add_threaded_test(info);
 
 	/* Check if channel was added successfully */
-	dtc = list_last_entry(&info->channels, struct dmatest_chan, node);
-
-	if (dtc->chan) {
+	if (!list_empty(&info->channels)) {
 		/*
 		 * if new channel was not successfully added, revert the
 		 * "test_channel" string to the name of the last successfully
 		 * added channel. exception for when users issues empty string
 		 * to channel parameter.
 		 */
+		dtc = list_last_entry(&info->channels, struct dmatest_chan, node);
 		if ((strcmp(dma_chan_name(dtc->chan), strim(test_channel)) != 0)
 		    && (strcmp("", strim(test_channel)) != 0)) {
 			ret = -EINVAL;