stats: return empty vector rather than NULL if stat_segment_dump_r() is run on an...

stats: return empty vector rather than NULL if stat_segment_dump_r() is run on an empty vector from ls

The return value in this function is initialized with 0, so if a vector of length 0 is passed
to stat_segment_dump_r, then this return value is never populated, resulting in inability
to distinguish between a successful dump of an empty vector and an error.

Solution: call vec_alloc(). As a side effect might get some trivial speed-up.

Type: fix
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
Change-Id: I33fefd801df457152e9ec257742305182e91f339