Deletion of unnecessary checks before specific function calls

The function "free" is documented in the way that no action shall occur for
a passed null pointer. It is therefore not needed that a function caller
repeats a corresponding check.
http://stackoverflow.com/questions/18775608/free-a-null-pointer-anyway-or-check-first

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>