[S390] kprobes: add parameter check to module_free()

When unregistering kprobes, kprobes calls module_free() and
always passes NULL for the mod parameter.  Add a check to
prevent NULL pointer dereferences.

See commit 740a8de0 for more details.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>