x86, SGI UV: uv_ptc_proc_write fix

Someone could write 0 bytes to /proc/sgi_uv/ptc_statistics,
causing
  optstr[count - 1] = '\0';
to write to who-knows-where.

(Andi Kleen noticed this need from a patch I sent for
 similar code in the ia64 world (sn2_ptc_proc_write()).)

(count less than zero is not possible here, as count is unsigned)

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>