iommu/omap: Check for NULL in iopte_free()

The iopte_free() function should check for NULL because
kmem_cache_free() will panic on NULL argument.
Signed-off-by: Zhouyi Zhou <yizhouzhou@ict.ac.cn>
Signed-off-by: Joerg Roedel <joro@8bytes.org>

drivers/iommu/omap-iommu.c

@@ -520,7 +520,8 @@ static void flush_iopte_range(u32 *first, u32 *last)
 static void iopte_free(u32 *iopte)
 {
 	/* Note: freed iopte's must be clean ready for re-use */
-	kmem_cache_free(iopte_cachep, iopte);
+	if (iopte)
+		kmem_cache_free(iopte_cachep, iopte);
 }
 
 static u32 *iopte_alloc(struct omap_iommu *obj, u32 *iopgd, u32 da)