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/2033Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>

src/runtime/os1_plan9.go

@@ -182,7 +182,7 @@ func exit(e int) {
 	} else {
 		// build error string
 		var tmp [32]byte
-		status = []byte(gostringnocopy(&itoa(tmp[:], uint64(e))[0]) + "\x00")
+		status = []byte(gostringnocopy(&itoa(tmp[:len(tmp)-1], uint64(e))[0]))
 	}
 	goexitsall(&status[0])
 	exits(&status[0])