[BUGFIX] Add missing check in PreparedStatement->execute()

When using prepared statements for inserts and updates, the
execute method currently fails due to a missing check on the
return value from $this->statement->result_metadata();

Resolves: #61229
Releases: 6.3, 6.2
Change-Id: Ida17f617915c36a0c2e91663eee64b4c358090e2
Reviewed-on: http://review.typo3.org/32457
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>