sh: Handle a NULL vma in __update_tlb() for the fast-path.

The TLB miss fast-path presently calls in to update_mmu_cache() to
set up the entry, and does so with a NULL vma. Check for vma validity
in the __update_tlb() ptrace checks.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
diff --git a/arch/sh/mm/tlb-sh4.c b/arch/sh/mm/tlb-sh4.c
index 81199f1..7d3c63e 100644
--- a/arch/sh/mm/tlb-sh4.c
+++ b/arch/sh/mm/tlb-sh4.c
@@ -22,7 +22,7 @@
 	/*
 	 * Handle debugger faulting in for debugee.
 	 */
-	if (current->active_mm != vma->vm_mm)
+	if (vma && current->active_mm != vma->vm_mm)
 		return;
 
 	local_irq_save(flags);