do not recycle empty dummy stacks

crates/wasmi/src/engine/mod.rs

@@ -292,7 +292,7 @@ impl EngineStacks {
 
     /// Disose and recycle the `stack`.
     pub fn recycle(&mut self, stack: Stack) {
-        if self.stacks.len() < self.keep {
+        if !stack.is_empty() && self.stacks.len() < self.keep {
             self.stacks.push(stack);
         }
     }

crates/wasmi/src/engine/stack/mod.rs

@@ -135,6 +135,15 @@ impl Stack {
         }
     }
 
+    /// Returns `true` if the [`Stack`] is empty.
+    ///
+    /// # Note
+    ///
+    /// Empty [`Stack`] instances are usually non-usable dummy instances.
+    pub(crate) fn is_empty(&self) -> bool {
+        self.values.is_empty()
+    }
+
     /// Push the given `frame` onto the call stack.
     ///
     /// # Note

crates/wasmi/src/engine/stack/values/mod.rs

@@ -98,6 +98,11 @@ impl ValueStack {
         }
     }
 
+    /// Returns `true` if the [`ValueStack`] is empty.
+    pub fn is_empty(&self) -> bool {
+        self.entries.capacity() == 0
+    }
+
     /// Creates a new empty [`ValueStack`].
     ///
     /// # Panics