runtime: terminate exit status buffer on Plan 9

The status buffer built by the exit function
was not nil-terminated.

Fixes #10789.

Change-Id: I2d34ac50a19d138176c4b47393497ba7070d5b61
Reviewed-on: https://go-review.googlesource.com/9953Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/os1_plan9.go

@@ -177,7 +177,7 @@ func exit(e int) {
 	} else {
 		// build error string
 		var tmp [32]byte
-		status = []byte(gostringnocopy(&itoa(tmp[:len(tmp)-1], uint64(e))[0]))
+		status = append(itoa(tmp[:len(tmp)-1], uint64(e)), 0)
 	}
 	goexitsall(&status[0])
 	exits(&status[0])