sql: fix string dequoting

Previously,

select "t1"."a" from (select "a" from "t") as "t1";

returned a result column name `t1` instead of `t1.a` because of
incorrect work of a dequoting function. The reason was that
previously sqlDequote() function finished its work when found the
first closing quote.

Old logic worked for simple selects where the column name doesn't
contain an explicit scan name ("a" -> a).
But for the sub-queries results sqlDequote() finished its work right
after the scan name ("t1"."a" -> t1). Now the function continues its
deqouting till it gets the null terminator at the end of the string.

Closes #7063

NO_DOC=don't change any public API, only a bug fix

Co-authored-by: Mergen Imeev <imeevma@gmail.com>