[PATCH] avoid out of bounds array reference in save_trace()

["Luck, Tony" <tony.luck@intel.com>]

ia64 found this the hard way (because we currently have a stub for
save_stack_trace() that does nothing). But it would be a good idea to 
be cautious in case a real save_stack_trace() bailed out with an
error before it set trace->nr_entries.

Signed-off-by: Tony Luck <tony.luck@intel.com>

---

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index 03c06af..429540c 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -387,7 +387,8 @@ static int save_trace(struct stack_trace *trace)
 	 * complete trace that maxes out the entries provided will be reported
 	 * as incomplete, friggin useless </rant>
 	 */
-	if (trace->entries[trace->nr_entries-1] == ULONG_MAX)
+	if (trace->nr_entries != 0 &&
+	    trace->entries[trace->nr_entries-1] == ULONG_MAX)
 		trace->nr_entries--;
 
 	trace->max_entries = trace->nr_entries;