perf top: Don't pass an ERR_PTR() directly to perf_session__delete()

While debugging a segfault on 'perf lock contention' without an available perf.data file I noticed that it was basically calling:

	perf_session__delete(ERR_PTR(-1))

Resulting in:

(gdb) run lock contention
Starting program: /root/bin/perf lock contention
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1". failed to open perf.data: No such file or directory (try 'perf record' first) Initializing perf session failed

Program received signal SIGSEGV, Segmentation fault.
0x00000000005e7515 in auxtrace__free (session=0xffffffffffffffff) at util/auxtrace.c:2858
2858 if (!session->auxtrace)
(gdb) p session
$1 = (struct perf_session *) 0xffffffffffffffff
(gdb) bt

  #0  0x00000000005e7515 in auxtrace__free (session=0xffffffffffffffff) at util/auxtrace.c:2858
  #1  0x000000000057bb4d in perf_session__delete (session=0xffffffffffffffff) at util/session.c:300
  #2  0x000000000047c421 in __cmd_contention (argc=0, argv=0x7fffffffe200) at builtin-lock.c:2161
  #3  0x000000000047dc95 in cmd_lock (argc=0, argv=0x7fffffffe200) at builtin-lock.c:2604
  #4  0x0000000000501466 in run_builtin (p=0xe597a8 <commands+552>, argc=2, argv=0x7fffffffe200) at perf.c:322
  #5  0x00000000005016d5 in handle_internal_command (argc=2, argv=0x7fffffffe200) at perf.c:375
  #6  0x0000000000501824 in run_argv (argcp=0x7fffffffe02c, argv=0x7fffffffe020) at perf.c:419
  #7  0x0000000000501b11 in main (argc=2, argv=0x7fffffffe200) at perf.c:535

(gdb)

So just set it to NULL after using PTR_ERR(session) to decode the error as perf_session__delete(NULL) is supported.

---
 tools/perf/builtin-top.c | 1 +
 1 file changed, 1 insertion(+)

index 1baa2acb3cedd989..ea8c7eca5eeedd76 100644
@@ -1805,6 +1805,7 @@ int cmd_top(int argc, const char **argv)
 	top.session = perf_session__new(NULL, NULL);
 	if (IS_ERR(top.session)) {
 		status = PTR_ERR(top.session);
+		top.session = NULL;
 		goto out_delete_evlist;
 	}
 

2.37.1