sd-event: make sure to create a signal queue for the right signal

sd-event: make sure to create a signal queue for the right signal

We should never access the "signal" part of the event source unless the
event source is actually for a signal. In this case it's a child pid
handler however, hence make sure to use the right signal.

This is a fix for PR #1177, which in turn was a fix for
9da4cb2be260ed123f2676cb85cb350c527b1492.