[BUGFIX] Use correct database connection for table in ReferenceIndex

Reusing the connection object for the sys_refindex table can fail
if the table that is being indexed is actually on another connection.
Select the proper connection before attempting to execute the query.

Change-Id: I4c031428f7405031aea6ef652c8b6f488f921228
Resolves: #81049
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/53131


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Henrik Elsner <helsner@dfau.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>