restore [o|i]ip zeroing to trust and fix '$get_clause_p'/3 (#2238)

src/machine/stack.rs

@@ -203,6 +203,12 @@ impl Stack {
         }
     }
 
+    pub(crate) fn top(&self) -> usize {
+        unsafe {
+            (*self.buf.ptr.get()) as usize - self.buf.base as usize
+        }
+    }
+
     pub(crate) fn allocate_or_frame(&mut self, num_cells: usize) -> usize {
         let frame_size = OrFrame::size_of(num_cells);