scsi: ufs: host: mediatek: Fix invalid access in vccqx handling

Add a NULL check before accessing the 'vccqx' pointer to prevent invalid
memory access. This ensures that the function safely handles cases where
'vccq' and 'vccq2' are not initialized, improving the robustness of the
power management code.

Signed-off-by: Alice Chao <alice.chao@mediatek.com>
Reviewed-by: Peter Wang <peter.wang@mediatek.com>
Signed-off-by: Peter Wang <peter.wang@mediatek.com>
Link: https://lore.kernel.org/r/20250811131423.3444014-11-peter.wang@mediatek.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: Alice Chao

drivers/ufs/host/ufs-mediatek.c

@@ -1629,6 +1629,9 @@ static void ufs_mtk_vccqx_set_lpm(struct ufs_hba *hba, bool lpm)
 {
 	struct ufs_vreg *vccqx = NULL;
 
+	if (!hba->vreg_info.vccq && !hba->vreg_info.vccq2)
+		return;
+
 	if (hba->vreg_info.vccq)
 		vccqx = hba->vreg_info.vccq;
 	else