[BUGFIX] Don't query database with non-numeric uids in RelationHandler

Check that a uid is numeric to prevent errors when querying an integer
database field with a string value like NEW123456789.

Resolves: #79455
Releases: master
Change-Id: I03a71f31c537e04bc05fcf13f7153c7f6adb3ae7
Reviewed-on: https://review.typo3.org/51396


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Manuel Selbach <manuel_selbach@yahoo.de>
Tested-by: Manuel Selbach <manuel_selbach@yahoo.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>