perf kmem: Fix segfault when invalid sort key is given

perf kmem: Fix segfault when invalid sort key is given

When it tries to free 'str', it was already updated by strsep() - so it
needs to save the original pointer.

  # perf kmem stat -s xxx,hit
    Error: Unknown --sort key: 'xxx'
  *** Error in `perf': free(): invalid pointer: 0x0000000000e9e7b6 ***
  ======= Backtrace: =========
  /usr/lib/libc.so.6(+0x7198e)[0x7fc7e6e0d98e]
  /usr/lib/libc.so.6(+0x76dee)[0x7fc7e6e12dee]
  /usr/lib/libc.so.6(+0x775cb)[0x7fc7e6e135cb]
  ./perf[0x44a1b5]
  ./perf[0x490b20]
  ./perf(parse_options_step+0x173)[0x491773]
  ./perf(parse_options_subcommand+0xa7)[0x491fb7]
  ./perf(cmd_kmem+0x2bc)[0x44ae4c]
  ./perf[0x47aa13]
  ./perf(main+0x60a)[0x427a9a]
  /usr/lib/libc.so.6(__libc_start_main+0xf0)[0x7fc7e6dbc800]
  ./perf(_start+0x29)[0x427bb9]

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1426145571-3065-2-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>