make z_stream::default() use follow the standard allocator preference

which is rust, if available, then C, if available, else None

zlib-rs/src/c_api.rs

@@ -40,7 +40,7 @@ pub type z_streamp = *mut z_stream;
 
 impl Default for z_stream {
     fn default() -> Self {
-        Self {
+        let mut stream = Self {
             next_in: core::ptr::null_mut(),
             avail_in: 0,
             total_in: 0,
@@ -49,13 +49,25 @@ impl Default for z_stream {
             total_out: 0,
             msg: core::ptr::null_mut(),
             state: core::ptr::null_mut(),
-            zalloc: Some(crate::allocate::zalloc_c),
-            zfree: Some(crate::allocate::zfree_c),
+            zalloc: None,
+            zfree: None,
             opaque: core::ptr::null_mut(),
             data_type: 0,
             adler: 0,
             reserved: 0,
+        };
+
+        #[cfg(feature = "rust-allocator")]
+        if stream.zalloc.is_none() || stream.zfree.is_none() {
+            stream.configure_default_rust_allocator()
         }
+
+        #[cfg(feature = "c-allocator")]
+        if stream.zalloc.is_none() || stream.zfree.is_none() {
+            stream.configure_default_c_allocator()
+        }
+
+        stream
     }
 }