fix bug in Stack::pop_frame

Author: Robbepop

crates/wasmi/src/engine/executor/handler/state.rs

@@ -273,7 +273,7 @@ impl Stack {
         instance: Inst,
     ) -> Option<(Ip, Sp, Mem0Ptr, Mem0Len, Inst)> {
         let (ip, start, changed_instance) = self.frames.pop()?;
-        let sp = self.values.sp(start);
+        let sp = self.values.sp_or_dangling(start);
         let (mem0, mem0_len, instance) = match changed_instance {
             Some(instance) => {
                 let (mem0, mem0_len) = extract_mem0(store, instance);
@@ -325,9 +325,20 @@ impl ValueStack {
     }
 
     fn sp(&mut self, start: usize) -> Sp {
+        debug_assert!(!self.cells.is_empty());
         Sp::new(&mut self.cells, start)
     }
 
+    fn sp_or_dangling(&mut self, start: usize) -> Sp {
+        match self.cells.is_empty() {
+            true => {
+                debug_assert_eq!(start, 0);
+                Sp::dangling()
+            }
+            false => self.sp(start),
+        }
+    }
+
     fn prepare_host_frame<'a>(
         &'a mut self,
         caller_start: usize,