ngql Variable undefined behaviors in yield clause

[czpmango]

(czp@nebula) [nba]> $var=go from "Tim Duncan" over * yield like._dst as dst;yield $var
+------+
| $var |
+------+
|      |
+------+
Got 1 rows (time spent 6724/7816 us)

(czp@nebula) [nba]> $var=go from "Tim Duncan" over * yield like._dst as dst;yield $var[0][1]
+--------------+
| $var[0][1]   |
+--------------+
| OUT_OF_RANGE |
+--------------+
Got 1 rows (time spent 5579/6662 us)

Whether to disable variable in yield clause?

[Shylock-Hg]

Better don't allow user to write variable expression.

[czpmango]

Better don't allow user to write variable expression.

ACK.

[CPWstatic]

These statements are illegal as we discussed offline.

[czpmango]

These statements are illegal as we discussed offline.

Fixed in #3271