eventlog: Fix off-by-one error in postIPE
We were missing the extra_comma from the calculation of the size of the
payload of postIPE. This was causing assertion failures when the event
would overflow the buffer by one byte, as ensureRoomForVariable event
would report there was enough space for n
bytes but then we would
write n + 1
bytes into the buffer.
Fixes #24287 (closed)