re-add UnwrapMutOrNull for linux

Signed-off-by: Walker Crouse <[email protected]>

zeroconf/src/ffi/mod.rs

@@ -37,6 +37,20 @@ impl<T> UnwrapOrNull<T> for Option<*const T> {
     }
 }
 
+/// Helper trait to unwrap a type to a `*mut T` or a null-pointer if not present.
+#[cfg(target_os = "linux")]
+pub trait UnwrapMutOrNull<T> {
+    /// Unwraps this type to `*mut T` or `ptr::null_mut()` if not present.
+    fn unwrap_mut_or_null(&mut self) -> *mut T;
+}
+
+#[cfg(target_os = "linux")]
+impl<T> UnwrapMutOrNull<T> for Option<*mut T> {
+    fn unwrap_mut_or_null(&mut self) -> *mut T {
+        self.unwrap_or_else(ptr::null_mut)
+    }
+}
+
 #[cfg(target_vendor = "apple")]
 pub(crate) mod bonjour {
     use crate::Result;