windows: return error if DecomposeCommandLine parameter contains NUL

DecomposeCommandLine is documented to use CommandLineToArgv, and the
CommandLineToArgvW system call inherently does not support strings with
internal NUL bytes. This CL changes DecomposeCommandLine to reject those
strings with an error instead of panicking.

Fixes golang/go#58817

Change-Id: I22a026bf2e69344a21f04849c50ba19b6e7b2007
Reviewed-on: https://go-review.googlesource.com/c/sys/+/487695
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>

windows/syscall_windows_test.go

@@ -626,6 +626,22 @@ func TestCommandLineRecomposition(t *testing.T) {
 			continue
 		}
 	}
+
+	// check that windows.DecomposeCommandLine returns error for strings with NUL
+	testsWithNUL := []string{
+		"\x00abcd",
+		"ab\x00cd",
+		"abcd\x00",
+		"\x00abcd\x00",
+		"\x00ab\x00cd\x00",
+		"\x00\x00\x00",
+	}
+	for _, test := range testsWithNUL {
+		_, err := windows.DecomposeCommandLine(test)
+		if err == nil {
+			t.Errorf("Failed to return error while decomposing %#q string with NUL inside", test)
+		}
+	}
 }
 
 func TestWinVerifyTrust(t *testing.T) {