srtpdec: fix "srtp-key" check

srtpdec: fix "srtp-key" check

The original code was:

if (!gst_structure_get (s, "srtp-key", GST_TYPE_BUFFER, &buf, NULL) || !buf) {
  goto error;
} else {
  stream->key = buf;
}

So use "srtp-key" if it is set so a non NULL buffer. The condition was
incorrectly inverted in ad7ffe64a66ab48d81671651031c449149db4973 to:

if (gst_structure_get (s, "srtp-key", GST_TYPE_BUFFER, &buf, NULL) || !buf) {
  stream->key = buf;
} ...

Fix the condition so it works as originally intended and avoid accessing
'buf' uninitialised.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4423>