syscall: return error instead of panicking in windows StartProcess

Fixes #11417

Change-Id: Iacea829a48b39df0a4f751b06b19e918fbb713d0
Reviewed-on: https://go-review.googlesource.com/11604Reviewed-by: Rob Pike <r@golang.org>

src/os/os_windows_test.go

@@ -117,3 +117,12 @@ func TestStatJunctionLink(t *testing.T) {
 		t.Fatalf("link should point to %v but points to %v instead", expected, got)
 	}
 }
+
+func TestStartProcessAttr(t *testing.T) {
+	p, err := os.StartProcess(os.Getenv("COMSPEC"), []string{"/c", "cd"}, new(os.ProcAttr))
+	if err != nil {
+		return
+	}
+	defer p.Wait()
+	t.Fatalf("StartProcess expected to fail, but succeeded.")
+}

src/syscall/exec_windows.go

@@ -244,6 +244,9 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 	if len(attr.Files) > 3 {
 		return 0, 0, EWINDOWS
 	}
+	if len(attr.Files) < 3 {
+		return 0, 0, EINVAL
+	}
 
 	if len(attr.Dir) != 0 {
 		// StartProcess assumes that argv0 is relative to attr.Dir,