audiolatency: Handle audio buffers with invalid duration

pipewiresrc outputs audio buffers without a valid duration, so we need
to calculate it manually in that case.

Upstream issue: pipewire/pipewire#1438

Part-of: <gstreamer/gst-plugins-bad!2444>
29 jobs for !2444 with backport-mr-2419-into-1.18-audiolatency-pipewire-fixes in 29 minutes and 30 seconds (queued for 7 seconds)
merge request