Fix a bug that no expression case doesn't return valid value

Here is a script that reproduces this problem:

     x = case
         when true; 1
         end
     p x # => main # 1 is expected

src/codegen.c

@@ -1458,7 +1458,7 @@ codegen(codegen_scope *s, node *tree, int val)
         int pos = cursp();
         genop(s, MKOP_A(OP_LOADNIL, cursp()));
         if (pos3) dispatch_linked(s, pos3);
-        pop();
+        if (head) pop();
         genop(s, MKOP_AB(OP_MOVE, cursp(), pos));
         push();
       }