rtpav1depay: Skip unexpected leading fragments

If a packet is starting with a leading fragment but we do not expect to
receive one, then skip over it to the next OBU.

Not doing so would cause parsing of the middle of an OBU, which would
most likely fail and cause unnecessary warning messages about a
corrupted stream.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1387>

net/rtp/src/av1/depay/imp.rs

@@ -33,6 +33,8 @@ struct State {
     marked_packet: bool,
     /// if the next output buffer needs the DISCONT flag set
     needs_discont: bool,
+    /// if we saw a valid OBU since the last reset
+    found_valid_obu: bool,
     /// holds data for a fragment
     obu_fragment: Option<(UnsizedObu, Vec<u8>)>,
 }
@@ -43,6 +45,7 @@ impl Default for State {
             last_timestamp: None,
             marked_packet: false,
             needs_discont: true,
+            found_valid_obu: false,
             obu_fragment: None,
         }
     }
@@ -291,6 +294,21 @@ impl RTPAv1Depay {
             let (element_size, is_last_obu) =
                 self.find_element_info(rtp, &mut reader, &aggr_header, idx)?;
 
+            if idx == 0 && aggr_header.leading_fragment {
+                if state.found_valid_obu {
+                    gst::error!(
+                        CAT,
+                        imp: self,
+                        "invalid packet: unexpected leading OBU fragment"
+                    );
+                }
+                reader
+                    .seek(SeekFrom::Current(element_size as i64))
+                    .map_err(err_flow!(self, buf_read))?;
+                idx += 1;
+                continue;
+            }
+
             let header_pos = reader.position();
             let mut bitreader = BitReader::endian(&mut reader, ENDIANNESS);
             let obu = UnsizedObu::parse(&mut bitreader).map_err(err_flow!(self, obu_read))?;
@@ -299,6 +317,8 @@ impl RTPAv1Depay {
                 .seek(SeekFrom::Start(header_pos))
                 .map_err(err_flow!(self, buf_read))?;
 
+            state.found_valid_obu = true;
+
             // ignore these OBU types
             if matches!(obu.obu_type, ObuType::TemporalDelimiter | ObuType::TileList) {
                 reader