ldd: treat nil pathname as self

This is a helpful shortcut for examining a test program's ldd output.

Signed-off-by: Ophestra <cat@gensokyo.uk>

ldd/exec.go

@@ -36,7 +36,19 @@ const (
 // [os.SyscallError] for fault creating the stdout pipe,
 // [container.StartError] for fault during either stage of container setup.
 // Otherwise, it passes through the return values of [Decoder.Decode].
-func Resolve(ctx context.Context, msg message.Msg, pathname *check.Absolute) ([]*Entry, error) {
+func Resolve(
+	ctx context.Context,
+	msg message.Msg,
+	pathname *check.Absolute,
+) ([]*Entry, error) {
+	if pathname == nil {
+		if p, err := os.Executable(); err != nil {
+			return nil, err
+		} else if pathname, err = check.NewAbs(p); err != nil {
+			return nil, err
+		}
+	}
+
 	c, cancel := context.WithTimeout(ctx, lddTimeout)
 	defer cancel()