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

tools/perf/builtin-report.c

@@ -304,7 +304,7 @@ static size_t hists__fprintf_nr_sample_events(struct hists *hists, struct report
 
 	if (rep->mem_mode) {
 		ret += fprintf(fp, "\n# Total weight : %" PRIu64, nr_events);
-		ret += fprintf(fp, "\n# Sort order   : %s", sort_order);
+		ret += fprintf(fp, "\n# Sort order   : %s", sort_order ? : default_mem_sort_order);
 	} else
 		ret += fprintf(fp, "\n# Event count (approx.): %" PRIu64, nr_events);
 	return ret + fprintf(fp, "\n#\n");

tools/perf/util/sort.h

@@ -44,6 +44,7 @@ extern struct sort_entry sort_dso_to;
 extern struct sort_entry sort_sym_from;
 extern struct sort_entry sort_sym_to;
 extern enum sort_type sort__first_dimension;
+extern const char default_mem_sort_order[];
 
 struct he_stat {
 	u64			period;