Gstreamer: gstreamer: webrtcbin accepts remote sdp without a=fingerprint due to inverted presence check

A logic vulnerability was found in GStreamer's webrtcbin component. The _check_sdp_crypto() function contains an inverted boolean condition that causes it to accept remote SDP offers or answers that lack the required a=fingerprint attribute, while incorrectly rejecting those that include it. An attacker with the ability to intercept and modify WebRTC signaling messages could exploit this to bypass the SDP-level DTLS certificate fingerprint binding, weakening defenses against man-in-the-middle attacks on media streams.