Fix an off-by-one in anyref stack manipulation

With more than two anyref stack arguments we were accidentally storing
the anyref values one higher in the stack than intended, so fix this
off-by-one by switching up some addition logic.

Author: alexcrichton

crates/anyref-xform/src/lib.rs

@@ -510,14 +510,14 @@ impl Transform<'_> {
                     // Store an anyref at an offset from our function's stack
                     // pointer frame.
                     let get_fp = builder.local_get(fp);
-                    next_stack_offset += 1;
-                    let (index, idx_local) = if next_stack_offset == 1 {
+                    let (index, idx_local) = if next_stack_offset == 0 {
                         (get_fp, fp)
                     } else {
                         let rhs = builder.i32_const(next_stack_offset);
                         let add = builder.binop(BinaryOp::I32Add, get_fp, rhs);
                         (builder.local_tee(scratch_i32, add), scratch_i32)
                     };
+                    next_stack_offset += 1;
                     let store = builder.table_set(self.table, index, local);
                     let get = builder.local_get(idx_local);
                     builder.with_side_effects(vec![store], get, Vec::new())