Minor changes for problems `cargo clippy` found

src/ffi.rs

@@ -159,7 +159,7 @@ extern "C" {
                                                     -> c_int;
 
     pub fn secp256k1_ecdsa_signature_serialize_der(cx: Context, output: *const c_uchar,
-                                                   out_len: *const size_t, sig: *const Signature)
+                                                   out_len: *mut size_t, sig: *const Signature)
                                                    -> c_int;
 
     pub fn secp256k1_ecdsa_recoverable_signature_parse_compact(cx: Context, sig: *mut RecoverableSignature,

src/macros.rs

@@ -36,6 +36,10 @@ macro_rules! impl_array_newtype {
             #[inline]
             /// Returns the length of the object as an array
             pub fn len(&self) -> usize { $len }
+
+            #[inline]
+            /// Returns whether the object as an array is empty
+            pub fn is_empty(&self) -> bool { false }
         }
 
         impl PartialEq for $thing {