fix typo in RTCP padding parsing

This was accidentally looking at the bottom bit of the V=2 field, so
it effectively never subtracted padding from the payload.

This bug didn't have any effect...yet. We currently only support
parsing the sender report. RFC 3550 says that SRs are sent
in "compound packets" that start with the SR and have at least one
other entry (a CNAME). Only the last packet within the compound packet
can have padding, so it's never come up in SR parsing. Fix this bug now
before we start parsing more things and run into trouble.

src/rtcp.rs

@@ -138,7 +138,7 @@ impl<'a> GenericPacket<'a> {
             ));
         }
         let (this, rest) = buf.split_at(len);
-        let padding_bit = this[0] & 0b00100_0000;
+        let padding_bit = this[0] & 0b0010_0000;
         if padding_bit != 0 {
             if raw_len == 0 {
                 return Err("RTCP packet has invalid combination of padding and len=0".to_owned());
@@ -209,4 +209,13 @@ mod tests {
         }
         assert_eq!(buf.len(), 0);
     }
+
+    #[test]
+    fn padding() {
+        let buf = b"\xa7\x00\x00\x02asdf\x00\x00\x00\x04rest";
+        let (pkt, rest) = GenericPacket::parse(buf).unwrap();
+        assert_eq!(pkt.count(), 7);
+        assert_eq!(&pkt.buf[4..pkt.payload_end], b"asdf");
+        assert_eq!(b"rest", rest);
+    }
 }