During spider query construction of certain cast functions, it
locates the last occurrence of a keyword in the output of the
Item::print() function and append from there to the constructed query
so far. For example, consider the following query

SELECT * FROM t2 ORDER BY CAST(c AS INET6);

It constructs the following query and executes it at the data
node (assuming the data node table is called t0).

select cast(t0.`c` as inet6) ``,t0.`c` `c` from `test`.`t1` t0 order by ``

When the construction has completed the initial part

select cast(t0.`c`

It then attempts to construct the " as inet6" part. To that end, it
calls print() on the Item_typecast_fbt corresponding to the cast item,
and obtains

cast(`test`.`t2`.`c` as inet6)

It then looks for " as ", and places cursor there for appending:

cast(`test`.`t2`.`c` as inet6)
                    ^

In this patch, if the search fails, i.e. there's no " as ...", we
make sure that the cursor is not placed before the beginning of the
string (out of bound).

We also relax the search from " as char" to " as " in the case of
CHAR_TYPECAST_FUNC, since there is more than one Item type with this
func type. For example, "AS INET6" is an Item_typecast_fbt which has
this func type.