sar/sadf: Don't reallocate buffers for activities not present in file

sar/sadf: Don't reallocate buffers for activities not present in file

Buffer reallocation would fail for activities which are not in the file
being read. This is because @nr2 = -1 and @nr2 is used to compute the
memory to allocate in reallocate_all_buffers():

SREALLOC(a->buf[j], void,
	 (size_t) a->msize * nr_realloc * (size_t) a->nr2);

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>