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 <namhyung@kernel.org>
CC: Arun Sharma <asharma@fb.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/1401335910-16832-26-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>

tools/perf/util/sort.c

@@ -1582,6 +1582,9 @@ void reset_output_field(void)
 	sort__has_sym = 0;
 	sort__has_dso = 0;
 
+	field_order = NULL;
+	sort_order = NULL;
+
 	reset_dimensions();
 	perf_hpp__reset_output_field();
 }