[BUGFIX] Fix return value of DatabaseConnection::sql_free_result()

In case of success the current implementation returns the result
of $res->free(). Unfortunately free() doesn't return anything.
That causes sql_free_result() to return NULL, which eventually
evaluates to FALSE.

Make sure the return value is correct and
return TRUE in case of success.

Resolves: #60457
Releases: 6.2, 6.3
Change-Id: I8756d4d2a321adc88738b9ac418207c63b312c86
Reviewed-on: https://review.typo3.org/31719
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>