perf tools: Reset output/sort order to default

When reset_output_field() is called, also reset field/sort order to
NULL so that it can have the default values. It's needed for testing.

Signed-off-by: Namhyung Kim <[e-mail]>
CC: Arun Sharma <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jiri Olsa <[e-mail]>

tools/perf/util/sort.c

···
 	sort__has_sym = 0;
 	sort__has_dso = 0;
 
+	field_order = NULL;
+	sort_order = NULL;
+
 	reset_dimensions();
 	perf_hpp__reset_output_field();
 }