perf tools: Fix parse_events_error dereferences

Parse errors can be reported in struct parse_events_error but the
pointer passed is optional and can be NULL.  Ensure it is not NULL
before dereferencing it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Link: http://lkml.kernel.org/r/1432040746-1755-4-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-events.c

@@ -1659,6 +1659,8 @@ void parse_events_evlist_error(struct parse_events_evlist *data,
 {
 	struct parse_events_error *err = data->error;
 
+	if (!err)
+		return;
 	err->idx = idx;
 	err->str = strdup(str);
 	WARN_ONCE(!err->str, "WARNING: failed to allocate error string");

tools/perf/util/parse-events.y

@@ -389,8 +389,10 @@ PE_NAME ':' PE_NAME
 	if (parse_events_add_tracepoint(list, &data->idx, $1, $3)) {
 		struct parse_events_error *error = data->error;
 
-		error->idx = @1.first_column;
-		error->str = strdup("unknown tracepoint");
+		if (error) {
+			error->idx = @1.first_column;
+			error->str = strdup("unknown tracepoint");
+		}
 		return -1;
 	}
 	$$ = list;