Fix clippy in mem.rs

Author: NobodyXu

src/mem.rs

@@ -142,7 +142,7 @@ impl Compress {
         // apparently 0-length compression requests which don't actually make
         // any progress are returned as BZ_PARAM_ERROR, which we don't want, to
         // just translate to a success here.
-        if input.len() == 0 && action == Action::Run {
+        if input.is_empty() && action == Action::Run {
             return Ok(Status::RunOk);
         }
         self.inner.raw.next_in = input.as_ptr() as *mut _;