srtpenc: Fix potential leak

When attempting to process a buffer after the rtcp session was closed the output
buffer memory would remain referenced.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6048>

subprojects/gst-plugins-bad/ext/srtp/gstsrtpenc.c

@@ -1149,6 +1149,7 @@ gst_srtp_enc_process_buffer (GstSrtpEnc * filter, GstPad * pad,
   if (filter->session == NULL) {
     /* The rtcp session disappeared (element shutting down) */
     GST_OBJECT_UNLOCK (filter);
+    gst_buffer_unmap (bufout, &mapout);
     ret = GST_FLOW_FLUSHING;
     goto fail;
   }