Fix a bug that if and no return value case can't return true clause value

Here is a script that reproduce this problem:

     x = if true
           1
         else
           case 2
           when 3
           end
           4
         end
     p x # => nil # 1 is expected

src/codegen.c

@@ -1462,8 +1462,13 @@ codegen(codegen_scope *s, node *tree, int val)
         genop(s, MKOP_AB(OP_MOVE, cursp(), pos));
         push();
       }
-      else if (pos3) {
-        dispatch_linked(s, pos3);
+      else {
+        if (pos3) {
+          dispatch_linked(s, pos3);
+        }
+        if (head) {
+          pop();
+        }
       }
     }
     break;