Cleanup vm stack on function return (#1880)

This cleans up the vm stack on a function return. This is needed because when errors are thrown, there may be still values on the stack that cannot be cleaned up via regular control flow. The change should have little performance impact, because the `Vec::truncate` function does not alter the capacity of the vector.

boa_engine/src/vm/mod.rs

@@ -1450,6 +1450,8 @@ impl Context {
             );
         }
 
+        let start_stack_size = self.vm.stack.len();
+
         while self.vm.frame().pc < self.vm.frame().code.code.len() {
             let result = if self.vm.trace {
                 let mut pc = self.vm.frame().pc;
@@ -1496,6 +1498,7 @@ impl Context {
             match result {
                 Ok(ShouldExit::True) => {
                     let result = self.vm.pop();
+                    self.vm.stack.truncate(start_stack_size);
                     return Ok((result, ReturnType::Normal));
                 }
                 Ok(ShouldExit::False) => {}
@@ -1543,6 +1546,7 @@ impl Context {
                         self.vm.frame_mut().finally_return = FinallyReturn::Err;
                         self.vm.push(e);
                     } else {
+                        self.vm.stack.truncate(start_stack_size);
                         return Err(e);
                     }
                 }
@@ -1576,6 +1580,8 @@ impl Context {
             return Ok((JsValue::undefined(), ReturnType::Normal));
         }
 
-        Ok((self.vm.pop(), ReturnType::Normal))
+        let result = self.vm.pop();
+        self.vm.stack.truncate(start_stack_size);
+        Ok((result, ReturnType::Normal))
     }
 }