resource_channels.c: Fix memory leak in ast_ari_channels_external_media.

resource_channels.c: Fix memory leak in ast_ari_channels_external_media.

Between ast_ari_channels_external_media(), external_media_rtp_udp(),
and external_media_audiosocket_tcp(), the `variables` structure being passed
around wasn't being cleaned up properly when there was a failure.

* In ast_ari_channels_external_media(), the `variables` structure is now
  defined with RAII_VAR to ensure it always gets cleaned up.

* The ast_variables_destroy() call was removed from external_media_rtp_udp().

* The ast_variables_destroy() call was removed from
  external_media_audiosocket_tcp(), its `endpoint` allocation was changed to
  to use ast_asprintf() as external_media_rtp_udp() does, and it now
  returns an error on failure.

* ast_ari_channels_external_media() now checks the new return code from
  external_media_audiosocket_tcp() and sets the appropriate error response.

Resolves: #1109