runtime: fix exit on Plan 9

Add a nil byte at the end of the itoa buffer,
before calling gostringnocopy. This prevents
gostringnocopy to read past the buffer size.

Change-Id: I87494a8dd6ea45263882536bf6c0f294eda6866d
Reviewed-on: https://go-review.googlesource.com/2033
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>