perf config: Handle NULL at perf_config_set__delete()

perf config: Handle NULL at perf_config_set__delete()

perf_config_set__delete() purge and free the config set that contains
all config key-value pairs.  But if the config set (i.e. 'set' variable
at the function) 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>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1465389413-8936-2-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>