drm/xe/guc: Fix dereference before NULL check

The pointer list->list is dereferenced before the NULL check.
Fix this by moving the NULL check outside the for loop, so that
the check is performed before the dereferencing.
The list->list pointer cannot be NULL so this has no effect on runtime.
It's just a correctness issue.

This issue was reported by Coverity Scan.
https://scan7.scan.coverity.com/#/project-view/51525/11354?selectedIssue=1600335

Fixes: 0f1fdf5 ("drm/xe/guc: Save manual engine capture into capture list")
Signed-off-by: Everest K.C. <everestkc@everestkc.com.np>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241023233356.5479-1-everestkc@everestkc.com.np
(cherry picked from commit 2aff81e)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

Author: everestkc

drivers/gpu/drm/xe/xe_guc_capture.c

@@ -1531,7 +1531,7 @@ read_reg_to_node(struct xe_hw_engine *hwe, const struct __guc_mmio_reg_descr_gro
 {
 	int i;
 
-	if (!list || list->num_regs == 0)
+	if (!list || !list->list || list->num_regs == 0)
 		return;
 
 	if (!regs)
@@ -1541,9 +1541,6 @@ read_reg_to_node(struct xe_hw_engine *hwe, const struct __guc_mmio_reg_descr_gro
 		struct __guc_mmio_reg_descr desc = list->list[i];
 		u32 value;
 
-		if (!list->list)
-			return;
-
 		if (list->type == GUC_STATE_CAPTURE_TYPE_ENGINE_INSTANCE) {
 			value = xe_hw_engine_mmio_read32(hwe, desc.reg);
 		} else {