dma-buf: Fix static checker warning

Here is the warning message:

	drivers/dma-buf/dma-buf.c:917 dma_buf_map_attachment()
	error: 'sg_table' dereferencing possible ERR_PTR()

Fix by adding error checking before dereferencing the pointer.

Fixes: ac80cd17 ("dma-buf: Clarify that dma-buf sg lists are page aligned")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jianxin Xiong <jianxin.xiong@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/398485/

drivers/dma-buf/dma-buf.c

@@ -908,7 +908,7 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
 	}
 
 #ifdef CONFIG_DMA_API_DEBUG
-	{
+	if (!IS_ERR(sg_table)) {
 		struct scatterlist *sg;
 		u64 addr;
 		int len;