dmaengine: dw-axi-dmac: Do not dereference NULL structure

If "vdesc" is NULL, it cannot be used with vd_to_axi_desc(). Leave
"bytes" unchanged at 0. Seen under GCC 13 with -Warray-bounds:

../drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c: In function 'dma_chan_tx_status':
../drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c:329:46: warning: array subscript 0 is outside array bounds of 'struct
virt_dma_desc[46116860184273879]' [-Warray-bounds=]
  329 |                 bytes = vd_to_axi_desc(vdesc)->length;
      |                                              ^~

Fixes: 8e55444 ("dmaengine: dw-axi-dmac: Support burst residue granularity")
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Cc: Vinod Koul <vkoul@kernel.org>
Cc: dmaengine@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230127223623.never.507-kees@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/dw-axi-dmac/dw-axi-dmac-platform.c

@@ -325,8 +325,6 @@ dma_chan_tx_status(struct dma_chan *dchan, dma_cookie_t cookie,
 		len = vd_to_axi_desc(vdesc)->hw_desc[0].len;
 		completed_length = completed_blocks * len;
 		bytes = length - completed_length;
-	} else {
-		bytes = vd_to_axi_desc(vdesc)->length;
 	}
 
 	spin_unlock_irqrestore(&chan->vc.lock, flags);