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 740a8de079 for more details.
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>