perf report: Fix rbtree bug

Ingo Molnar reported:

> FYI, i just got this crash (segfault) in perf report after
> collecting a long profile from Xorg:
>
> Starting program: /home/mingo/tip/Documentation/perf_counter/perf report
> [Thread debugging using libthread_db enabled]
> Detaching after fork from child process 20008.
> [New Thread 0x7f92fd62a6f0 (LWP 20005)]
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x000000000041031a in __rb_erase_color (node=0x142c0901, parent=0x0,
> root=0x881918)
>     at util/rbtree.c:143
> 143			if (parent->rb_left == node)

It was a problem introduced in this cset:

 perf report: Fix comm sorting - 8229289b

This patch should fix it.
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephane Eranian <eranian@googlemail.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/perf_counter/builtin-report.c

@@ -699,17 +699,18 @@ static void output__resort(void)
 {
 	struct rb_node *next;
 	struct hist_entry *n;
+	struct rb_root *tree = &hist;
 
 	if (sort__need_collapse)
-		next = rb_first(&collapse_hists);
-	else
-		next = rb_first(&hist);
+		tree = &collapse_hists;
+
+	next = rb_first(tree);
 
 	while (next) {
 		n = rb_entry(next, struct hist_entry, rb_node);
 		next = rb_next(&n->rb_node);
 
-		rb_erase(&n->rb_node, &hist);
+		rb_erase(&n->rb_node, tree);
 		output__insert_entry(n);
 	}
 }