perf tools: Don't allow empty argument for field-separator

Both 'perf diff' and 'perf mem' have 'field-separator' option, which
causes segfault if passed with empty string. This patch uses previously
introduced 'OPT_STRING_NOEMPTY' option macro to prevent fault.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: pi3orama@163.com
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Link: http://lkml.kernel.org/r/1426820272-23302-1-git-send-email-wangnan0@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>