runtime: fix sprintf index out of bounds

Breaks on Plan 9, apparently.
The other systems must not run sprintf during all.bash.
I'd write a test but it's all going away.

TBR=r
CC=0intro, golang-codereviews
https://golang.org/cl/133260044

src/pkg/runtime/print1.go

@@ -46,7 +46,7 @@ func printf(s *byte) {
 // It has the same problem as goprintf.
 //go:nosplit
 func snprintf(dst *byte, n int32, s *byte) {
-	buf := (*[1 << 30]byte)(unsafe.Pointer(dst))[0:0:n]
+	buf := (*[1 << 30]byte)(unsafe.Pointer(dst))[0:n:n]
 
 	gp := getg()
 	gp.writebuf = buf[0:0 : n-1] // leave room for NUL, this is called from C