locking/lockdep: Fix possible NULL deref

We can't invalidate xhlocks when we've not yet allocated any.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Fixes: f52be570 ("locking/lockdep: Untangle xhlock history save/restore from task independence")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -4790,6 +4790,7 @@ void lockdep_invariant_state(bool force)
 	 * Verify the former, enforce the latter.
 	 */
 	WARN_ON_ONCE(!force && current->lockdep_depth);
+	if (current->xhlocks)
 		invalidate_xhlock(&xhlock(current->xhlock_idx));
 }