perf tools: Remove (null) value of "Sort order" for perf mem report

When '--sort' is not set, 'perf mem report" will print a null pointer as
the output value of sort order, so fix it.

Example:

Before this patch:

 $ perf mem report
 # To display the perf.data header info, please use --header/--header-only options.
 #
 # Samples: 18  of event 'cpu/mem-loads/pp'
 # Total weight : 188
 # Sort order   : (null)
 #
 ...

After this patch:

 $ perf mem report
 # To display the perf.data header info, please use --header/--header-only options.
 #
 # Samples: 18  of event 'cpu/mem-loads/pp'
 # Total weight : 188
 # Sort order   : local_weight,mem,sym,dso,symbol_daddr,dso_daddr,snoop,tlb,locked
 #
 ...

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1427082605-12881-1-git-send-email-yunlong.song@huawei.com


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