Fix use-after-free when ending a transaction with registered users

[mariuz]

Reading a WITH TIME ZONE column and then disconnecting raises
EObjectCheck: Object reference is Nil from inside
TFBTransaction.DoDefaultTransactionEnd, reached via
TFBAttachment.EndAllTransactions during Disconnect.

Cause

Reading a time zone value creates TFB30TimeZoneServices, which starts an
internal transaction for the rdb$time_zone_util lookups and registers itself
on it:

FTransaction := FAttachment.StartTransaction([...], taCommit);
(FTransaction as TFBTransaction).AddObject(self);

At disconnect, EndAllTransactions calls DoDefaultTransactionEnd on that
transaction. Its loop calls TransactionEnding on each registered user, and
the time zone services clears its own reference:

(FTransaction as TFBTransaction).Remove(self);
FTransaction := nil;

TInterfaceOwner stores raw TInterfacedObject pointers rather than counted
references, and EndAllTransactions iterates them without taking one, so that
was the last reference to the transaction. The object is destroyed while
DoDefaultTransactionEnd is still executing, and the Commit(Force) at the
end of that same method then runs on freed memory.

Fix

Hold an ITransaction reference to self for the duration of the method, so a
user's TransactionEnding cannot destroy the transaction out from under the
Commit/Rollback that follows.

Reproduction

Against Firebird 6.0.0, with a table holding an integer, a plain TIMESTAMP,
a TIMESTAMP WITH TIME ZONE and a TIME WITH TIME ZONE; each case opens one
TIBQuery, closes it, commits, then disconnects:

selected column	before	after
integer	disconnects cleanly	disconnects cleanly
TIMESTAMP	disconnects cleanly	disconnects cleanly
TIMESTAMP WITH TIME ZONE	EObjectCheck	disconnects cleanly
TIME WITH TIME ZONE	EObjectCheck	disconnects cleanly

It is not schema qualification, not the prepared statement (unpreparing first
does not help), and not global temporary tables; freeing the query and the
transaction before disconnecting does not help either, which is what pointed at
the attachment rather than the query.

Found while adding Firebird 4/5/6 support to Marathon, where it also surfaced
through the profiler: PLG$PROF_SESSIONS has TIMESTAMP WITH TIME ZONE
columns, so reading it made the following disconnect fault.