singlesocket: fix the 'sincebefore' placement

The variable wasn't properly reset within the loop and thus could remain
set for sockets that hadn't been set before and miss notifying the app.

This is a follow-up to 4c35574b (shipped in curl 7.64.0)

Reported-by: buzo-ffm on github
Detected-by: Jan Alexander Steffens
Fixes #3585
Closes #3589