freebsd11_network: Handle NULL gracefully instead of faulting.

FreeBSD does not have these checks, but drivers seem to expect that they
can call these functions with NULL and not crash.

Fixes a number of boot-failure tickets (and makes it possible for me at least
to test drivers without rebooting from KDL every failure), though of course the
drivers themselves will still not work.

src/libs/compat/freebsd11_network/callout.cpp

@@ -212,6 +212,11 @@ _callout_stop_safe(struct callout *c, int safe)
 {
 	MutexLocker locker(sLock);
 
+	if (c == NULL) {
+		printf("_callout_stop_safe called with NULL callout");
+		return 0;
+	}
+
 	TRACE("_callout_stop_safe %p, func %p, arg %p\n", c, c->c_func, c->c_arg);
 
 	if (c->due <= 0)

src/libs/compat/freebsd11_network/taskqueue.c

@@ -247,6 +247,11 @@ taskqueue_drain(struct taskqueue *taskQueue, struct task *task)
 {
 	cpu_status status;
 
+	if (taskQueue == NULL) {
+		printf("taskqueue_drain called with NULL taskqueue");
+		return;
+	}
+
 	tq_lock(taskQueue, &status);
 	while (task->ta_pending != 0) {
 		tq_unlock(taskQueue, status);