[bindings] Allow cloning opaque types when inner is NULL

Previously we'd segfault trying to deref the NULL page, but there
is no reason to not simply clone by creating another opaque instance
with a null inner. This comes up specifically when cloning
ChannelSigners as the pubkeys instance is NULL on construction
before get_pubkeys is called.

Author: TheBlueMatt

c-bindings-gen/src/main.rs

@@ -559,7 +559,8 @@ fn writeln_opaque<W: std::io::Write>(w: &mut W, ident: &syn::Ident, struct_name:
 		writeln!(w, "impl Clone for {} {{", struct_name).unwrap();
 		writeln!(w, "\tfn clone(&self) -> Self {{").unwrap();
 		writeln!(w, "\t\tSelf {{").unwrap();
-		writeln!(w, "\t\t\tinner: Box::into_raw(Box::new(unsafe {{ &*self.inner }}.clone())),").unwrap();
+		writeln!(w, "\t\t\tinner: if self.inner.is_null() {{ std::ptr::null_mut() }} else {{").unwrap();
+		writeln!(w, "\t\t\t\tBox::into_raw(Box::new(unsafe {{ &*self.inner }}.clone())) }},").unwrap();
 		writeln!(w, "\t\t\tis_owned: true,").unwrap();
 		writeln!(w, "\t\t}}\n\t}}\n}}").unwrap();
 		writeln!(w, "#[allow(unused)]").unwrap();
@@ -569,7 +570,7 @@ fn writeln_opaque<W: std::io::Write>(w: &mut W, ident: &syn::Ident, struct_name:
 		writeln!(w, "}}").unwrap();
 		writeln!(w, "#[no_mangle]").unwrap();
 		writeln!(w, "pub extern \"C\" fn {}_clone(orig: &{}) -> {} {{", struct_name, struct_name, struct_name).unwrap();
-		writeln!(w, "\t{} {{ inner: Box::into_raw(Box::new(unsafe {{ &*orig.inner }}.clone())), is_owned: true }}", struct_name).unwrap();
+		writeln!(w, "\torig.clone()").unwrap();
 		writeln!(w, "}}").unwrap();
 	}