perf probe: Fix to clear tev->nargs in clear_probe_trace_event()

Since add_probe_trace_event() can reuse tf->tevs[i] after calling
clear_probe_trace_event(), this can make perf-probe crash if the 1st
attempt of probe event finding fails to find an event argument, and the
2nd attempt fails to find probe point.

E.g.
  $ perf probe -D "task_pid_nr tsk"
  Failed to find 'tsk' in this function.
  Failed to get entry address of warn_bad_vsyscall
  Segmentation fault (core dumped)

Committer testing:

After the patch:

  $ perf probe -D "task_pid_nr tsk"
  Failed to find 'tsk' in this function.
  Failed to get entry address of warn_bad_vsyscall
  Failed to get entry address of signal_fault
  Failed to get entry address of show_signal
  Failed to get entry address of umip_printk
  Failed to get entry address of __bad_area_nosemaphore
  <SNIP>
  Failed to get entry address of sock_set_timeout
  Failed to get entry address of tcp_recvmsg
  Probe point 'task_pid_nr' not found.
    Error: Failed to add events.
  $

Fixes: 092b1f0b5f ("perf probe: Clear probe_trace_event when add_probe_trace_event() fails")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lore.kernel.org/lkml/156856587999.25775.5145779959474477595.stgit@devnote2
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -2331,6 +2331,7 @@ void clear_probe_trace_event(struct probe_trace_event *tev)
 		}
 	}
 	zfree(&tev->args);
+	tev->nargs = 0;
 }
 
 struct kprobe_blacklist_node {