[release-branch.go1.4] runtime: don't panic when given a callback with no input params on windows

Fixes #9871 for Go 1.4.

Change-Id: I550a5bdb29e9a872652e0dd468a434227d7d9502
Reviewed-on: https://go-review.googlesource.com/4937
Run-TryBot: Minux Ma <minux@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/runtime/syscall_windows.go

@@ -54,11 +54,13 @@ func compileCallback(fn eface, cleanstack bool) (code uintptr) {
 		panic("compilecallback: output parameter size is wrong")
 	}
 	argsize := uintptr(0)
-	for _, t := range (*[1024](*_type))(unsafe.Pointer(&ft.in[0]))[:len(ft.in)] {
-		if (*t).size > uintptrSize {
-			panic("compilecallback: input parameter size is wrong")
+	if len(ft.in) > 0 {
+		for _, t := range (*[1024](*_type))(unsafe.Pointer(&ft.in[0]))[:len(ft.in)] {
+			if (*t).size > uintptrSize {
+				panic("compilecallback: input parameter size is wrong")
+			}
+			argsize += uintptrSize
 		}
-		argsize += uintptrSize
 	}
 
 	lock(&cbs.lock)

src/runtime/syscall_windows_test.go

@@ -533,3 +533,11 @@ func main() {
 	println(z)
 }
 `
+
+func TestCallbackWithNoInputParameters(t *testing.T) {
+	// Test that NewCallback and NewCallbackCDecl can accept functions without
+	// input parameters, see issue 9871.
+	cb := func() uintptr { return 0 }
+	_ = syscall.NewCallback(cb)
+	_ = syscall.NewCallbackCDecl(cb)
+}