signal: Don't send signals to tasks that don't exist

Recently syzbot reported crashes in send_sigio_to_task and
send_sigurg_to_task in linux-next.  Despite finding a reproducer
syzbot apparently did not bisected this or otherwise track down the
offending commit in linux-next.

I happened to see this report and examined the code because I had
recently changed these functions as part of making PIDTYPE_TGID a real
pid type so that fork would does not need to restart when receiving a
signal.  By examination I see that I spotted a bug in the code
that could explain the reported crashes.

When I took Oleg's suggestion and optimized send_sigurg and send_sigio
to only send to a single task when type is PIDTYPE_PID or PIDTYPE_TGID
I failed to handle pids that no longer point to tasks.  The macro
do_each_pid_task simply iterates for zero iterations.  With pid_task
an explicit NULL test is needed.

Update the code to include the missing NULL test.

Fixes: 019191342f ("signal: Use PIDTYPE_TGID to clearly store where file signals will be sent")
Reported-by: syzkaller-bugs@googlegroups.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/fcntl.c

@@ -791,6 +791,7 @@ void send_sigio(struct fown_struct *fown, int fd, int band)
 	if (type <= PIDTYPE_TGID) {
 		rcu_read_lock();
 		p = pid_task(pid, PIDTYPE_PID);
+		if (p)
 			send_sigio_to_task(p, fown, fd, band, type);
 		rcu_read_unlock();
 	} else {
@@ -830,6 +831,7 @@ int send_sigurg(struct fown_struct *fown)
 	if (type <= PIDTYPE_TGID) {
 		rcu_read_lock();
 		p = pid_task(pid, PIDTYPE_PID);
+		if (p)
 			send_sigurg_to_task(p, fown, type);
 		rcu_read_unlock();
 	} else {