staging/sep: Check pointers before dereferencing (fix smatch warning)

smatch complains about two dereferenced before check issues:

sep_main.c:2898 sep_free_dma_tables_and_dcb() warn: variable dereferenced before check
'dma_ctx' (see line 2885)
sep_main.c:2898 sep_free_dma_tables_and_dcb() warn: variable dereferenced before check
'*dma_ctx' (see line 2885)

-> Move the checks to the top, but keep the semantics.
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/sep/sep_main.c

@@ -2880,6 +2880,8 @@ static int sep_free_dma_tables_and_dcb(struct sep_device *sep, bool isapplet,
 
 	dev_dbg(&sep->pdev->dev, "[PID%d] sep_free_dma_tables_and_dcb\n",
 					current->pid);
+	if (!dma_ctx || !*dma_ctx) /* nothing to be done here*/
+		return 0;
 
 	if (((*dma_ctx)->secure_dma == false) && (isapplet == true)) {
 		dev_dbg(&sep->pdev->dev, "[PID%d] handling applet\n",
@@ -2895,8 +2897,7 @@ static int sep_free_dma_tables_and_dcb(struct sep_device *sep, bool isapplet,
 		 * Go over each DCB and see if
 		 * tail pointer must be updated
 		 */
-		for (i = 0; dma_ctx && *dma_ctx &&
-			i < (*dma_ctx)->nr_dcb_creat; i++, dcb_table_ptr++) {
+		for (i = 0; i < (*dma_ctx)->nr_dcb_creat; i++, dcb_table_ptr++) {
 			if (dcb_table_ptr->out_vr_tail_pt) {
 				pt_hold = (unsigned long)dcb_table_ptr->
 					out_vr_tail_pt;