perf config: Handle the error when config set is NULL at collect_config()

collect_config() collect all config key-value pairs from config files
and put each config info in config set.  But if config set (i.e. 'set'
variable at collect_config()) is NULL, this is wrong so handle it.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1465210380-26749-4-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/config.c

@@ -607,8 +607,12 @@ static int collect_config(const char *var, const char *value,
 	struct perf_config_section *section = NULL;
 	struct perf_config_item *item = NULL;
 	struct perf_config_set *set = perf_config_set;
-	struct list_head *sections = &set->sections;
+	struct list_head *sections;
 
+	if (set == NULL)
+		return -1;
+
+	sections = &set->sections;
 	key = ptr = strdup(var);
 	if (!key) {
 		pr_debug("%s: strdup failed\n", __func__);