lockdep: Avoid out of bounds array reference in save_trace()

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>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: luming.yu@intel.com
LKML-Reference: <4b2024d085302c2a2@agluck-desktop.sc.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>