[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>

arch/s390/kernel/module.c

@@ -55,8 +55,10 @@ void *module_alloc(unsigned long size)
 /* Free memory returned from module_alloc */
 void module_free(struct module *mod, void *module_region)
 {
-	vfree(mod->arch.syminfo);
-	mod->arch.syminfo = NULL;
+	if (mod) {
+		vfree(mod->arch.syminfo);
+		mod->arch.syminfo = NULL;
+	}
 	vfree(module_region);
 }