baseparse: Always clear drain flag before pulling
In pull mode, each pull is unique. A following pull can be well inside the range even if the previous one wasn't. Fix this my moving the drain flag right before the pull.
This avoids passing a bad drain flag to parsers, which may endup truncate buffers causing data corruption.
Fixes gst-plugins-bad#1275 (closed)
cc @thaytan
Edited by GStreamer Release Monkey