app-layer/pd: only consider actual available data

app-layer/pd: only consider actual available data

For size limit checks consider only available data at the stream start
and before any GAPS.

The old check would consider too much data if there were temporary gaps,
like when a data packet was in-window but (far) ahead of the expected
segment.

(cherry picked from commit 7a114e506a27fcb2a3b5ed28b1c10fe100cf78c6)