[PATCH]highmem_32.c: add argument pointer checking

["Figo.zhang" <figo1802@gmail.com>]

It had better add argument pointer checking.

If any guys write driver want to alloc hightmem and pass a no-initial pointer,
it would be crashed.

Signed-off-by: Figo.zhang <figo1802@gmail.com>
---  
arch/x86/mm/highmem_32.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/arch/x86/mm/highmem_32.c b/arch/x86/mm/highmem_32.c
index 58f621e..e52e1a9 100644
--- a/arch/x86/mm/highmem_32.c
+++ b/arch/x86/mm/highmem_32.c
@@ -31,6 +31,7 @@ void *kmap_atomic_prot(struct page *page, enum km_type type, pgprot_t prot)
 {
 	enum fixed_addresses idx;
 	unsigned long vaddr;
+	BUG_ON(!page);
 
 	/* even !CONFIG_PREEMPT needs this, for in_atomic in do_page_fault */
 	pagefault_disable();
@@ -58,6 +59,9 @@ void kunmap_atomic(void *kvaddr, enum km_type type)
 	unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;
 	enum fixed_addresses idx = type + KM_TYPE_NR*smp_processor_id();
 
+	if(!kvaddr)
+		return;
+
 	/*
 	 * Force other mappings to Oops if they'll try to access this pte
 	 * without first remap it.  Keeping stale mappings around is a bad idea