srtpdec: fix Got data flow before segment event

srtpdec: fix Got data flow before segment event

A race condition can occur in `srtpdec` during the READY -> NULL transition:
an RTCP buffer can make its way to `gst_srtp_dec_chain` while the element is
partially stopped, resulting in the following critical warning:

> Got data flow before segment event

The problematic sequence is the following:

1. An RTCP buffer is being handled by the chain function for the
   `rtcp_sinkpad`. Since, this is the first buffer, we try pushing the sticky
   events to `rtcp_srcpad`.
2. At the same moment, the element is being transitioned from PAUSED to READY.
3. While checking and pushing the sticky events for `rtcp_srcpad`, we reach the
   Segment event. For this, we try to get it from the "otherpad", in this case
   `rtp_srcpad`. In the problematic case, `rtp_srcpad` has already been
   deactivated so its sticky events have been cleared. We won't be pushing any
   Segment event to `rtcp_srcpad`.
4. We return to the chain function for `rtcp_sinkpad` and try pushing the
   buffer to `rtcp_srcpad` for which deactivation hasn't started yet, hence the
   "Got data flow before segment event".

This commit:

- Adds a boolean return value to `gst_srtp_dec_push_early_events`: in case the
  Segment event can't be retrieved, `gst_srtp_dec_chain` can return  an error
  instead of calling `gst_pad_push`.
- Replaces the obsolete `gst_pad_set_caps` with `gst_pad_push_event`. The
  additional preconditions checked by previous function are guaranteed here
  since we push a fixed Caps which was built in the same function.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4860>