runtime: simplify signalstack by dropping nil as argument

Change the two calls to signalstack(nil) to inline the code
instead (it's two lines).

Change-Id: Ie92a05494f924f279e40ac159f1b677fda18f281
Reviewed-on: https://go-review.googlesource.com/29854
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/os_netbsd.go

@@ -192,7 +192,8 @@ func newosproc(mp *m, stk unsafe.Pointer) {
 // At this point all signals are blocked, so there is no race.
 //go:nosplit
 func netbsdMstart() {
-	signalstack(nil)
+	st := stackt{ss_flags: _SS_DISABLE}
+	sigaltstack(&st, nil)
 	mstart()
 }
 

src/runtime/signal_unix.go

@@ -624,7 +624,8 @@ func minitSignalMask() {
 //go:nosplit
 func unminitSignals() {
 	if getg().m.newSigstack {
-		signalstack(nil)
+		st := stackt{ss_flags: _SS_DISABLE}
+		sigaltstack(&st, nil)
 	}
 }
 
@@ -645,17 +646,10 @@ func setGsignalStack(st *stackt) {
 }
 
 // signalstack sets the current thread's alternate signal stack to s.
-// If s is nil, the current thread's alternate signal stack is disabled.
 //go:nosplit
 func signalstack(s *stack) {
-	var st stackt
-	if s == nil {
-		st.ss_flags = _SS_DISABLE
-	} else {
-		setSignalstackSP(&st, s.lo)
-		st.ss_size = s.hi - s.lo
-		st.ss_flags = 0
-	}
+	st := stackt{ss_size: s.hi - s.lo}
+	setSignalstackSP(&st, s.lo)
 	sigaltstack(&st, nil)
 }