ovs-thread: Fix crash by making count_cpu_count() return type a signed int.

ofproto_set_threads() uses the calculation MAX(count_cpu_cores() - 2, 1)
to decide on the default thread count.  However, count_cpu_cores() returns
0 if it can't count the number of cores, or 1 if there's only one core,
and that causes the calculation to come out as UINT_MAX-2 or UINT_MAX-1,
respectively, which causes a memory allocation failure later.

There are other ways to fix this problem, too, of course.

Signed-off-by: Ben Pfaff <blp@nicira.com>