[BUGFIX] fetch_field_redirect returns object but not array

Method sql_field_type was rewritten with the mysqli switch for
compatibility with the old behavior. The introduced call to
fetch_field_redirect returns an object, but the result is
handled as an array, leading to warnings.

Change-Id: I73c434e8ce8086f4e60e119f4c2e2d1e2c9c7d20
Fixes: #46806
Related: #36419
Reviewed-on: https://review.typo3.org/19448
Reviewed-by: Philipp Gampe
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn