fix #154

[rtbo]

diff of code generation:

diff -ur gen/previous/randr.rs gen/current/randr.rs
--- gen/previous/randr.rs   2022-02-13 09:29:55.510376289 +0100
+++ gen/current/randr.rs    2022-02-13 09:30:25.204064677 +0100
@@ -5169,14 +5169,17 @@
     }

     pub fn data<P: PropEl>(&self) -> &[P] {
-        assert_eq!(
-            self.format(),
-            P::FORMAT,
-            "mismatched format of randr::GetOutputPropertyReply::data"
-        );
         unsafe {
-            let offset = 32usize;
             let len = ((self.num_items() as usize) * ((self.format() as usize) / 8usize));
+            if len == 0 {
+                return &[];
+            }
+            assert_eq!(
+                self.format(),
+                P::FORMAT,
+                "mismatched format of randr::GetOutputPropertyReply::data"
+            );
+            let offset = 32usize;
             let len = len / std::mem::size_of::<P>();
             let ptr = self.wire_ptr().add(offset) as *const P;
             std::slice::from_raw_parts(ptr, len)
@@ -9739,14 +9742,17 @@
     }

     pub fn data<P: PropEl>(&self) -> &[P] {
-        assert_eq!(
-            self.format(),
-            P::FORMAT,
-            "mismatched format of randr::GetProviderPropertyReply::data"
-        );
         unsafe {
-            let offset = 32usize;
             let len = ((self.num_items() as usize) * ((self.format() as usize) / 8usize));
+            if len == 0 {
+                return &[];
+            }
+            assert_eq!(
+                self.format(),
+                P::FORMAT,
+                "mismatched format of randr::GetProviderPropertyReply::data"
+            );
+            let offset = 32usize;
             let len = len / std::mem::size_of::<P>();
             let ptr = self.wire_ptr().add(offset) as *const P;
             std::slice::from_raw_parts(ptr, len)
diff -ur gen/previous/xproto.rs gen/current/xproto.rs
--- gen/previous/xproto.rs  2022-02-13 09:29:58.210406347 +0100
+++ gen/current/xproto.rs   2022-02-13 09:30:27.964100206 +0100
@@ -13744,14 +13744,17 @@
     /// The value of the property. The type is known only at runtime through the [format] field.
     /// It must be casted to the proper type accordingly with the `P` type parameter.
     pub fn value<P: PropEl>(&self) -> &[P] {
-        assert_eq!(
-            self.format(),
-            P::FORMAT,
-            "mismatched format of xproto::GetPropertyReply::value"
-        );
         unsafe {
-            let offset = 32usize;
             let len = ((self.value_len() as usize) * ((self.format() as usize) / 8usize));
+            if len == 0 {
+                return &[];
+            }
+            assert_eq!(
+                self.format(),
+                P::FORMAT,
+                "mismatched format of xproto::GetPropertyReply::value"
+            );
+            let offset = 32usize;
             let len = len / std::mem::size_of::<P>();
             let ptr = self.wire_ptr().add(offset) as *const P;
             std::slice::from_raw_parts(ptr, len)