ImmediateAlertService: fix latent bug

[DavisNT]

Include null terminator in the bytes copied.
Set notif.size like it is done in AlertNotificationService.cpp and AlertNotificationClient.cpp.

[github-actions]

Build size and comparison to main:

Section	Size	Difference
text	374512B	-16B
data	948B	0B
bss	63488B	0B

[FintasticMan]

I'm fine with this implementation, but I think that it might be good to also use std::min to avoid overflowing the buffer. (Of course that isn't possible with the current notification strings, but it makes the code more clear if we do that)

Because there are some inconsistencies and incorrect handlings of the notification buffer elsewhere in the code, I'm thinking of adding a constructor to the struct that makes sure it's written correctly. In the meantime, this is a good fix.

[DavisNT]

@FintasticMan Thanks for the review!
In this particular case the alertString is one of string constants defined in lines 20-26 of ImmediateAlertService.cpp and is never longer than 12 characters (unless someone would modify this code).

In general constructor for NotificationManager::Notification that checks for buffer overflows sounds like a very good idea. 👍

P.S. Would you like me to implement this constructor?

[FintasticMan]

I was planning on doing it, but if you want to, go ahead!