[BUGFIX] sql_free_result does not work with all allowed types

sql_free_result throws a fatal error when called with a
boolean. According to the method description boolean is an
allowed type. Check if $res is an object.

Change-Id: I6f7cdbb42c07869a320510e9b0b779f2b7cf6b70
Resolves: #50378
Releases: 6.2, 6.1
Reviewed-on: https://review.typo3.org/22594
Reviewed-by: Dan Untenzu
Tested-by: Dan Untenzu
Reviewed-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Reviewed-by: Markus Klein
Tested-by: Markus Klein