Allow PLC to function properly when channels use SLIN for audio.

If a channel involved in a bridge was using SLIN audio, then translation
paths were not guaranteed to be set up properly since in all likelihood
the number of translation steps was only 1.

This patch enforces the transcode_via_slin behavior if transcode_via_slin
or generic_plc is enabled and one of the formats to make compatible is
SLIN.

AST-352



git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@278618 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/channel.c

@@ -3768,6 +3768,7 @@ int ast_channel_make_compatible(struct ast_channel *chan, struct ast_channel *pe
 {
 	int src;
 	int dst;
+	int use_slin;
 
 	if (chan->readformat == peer->writeformat && chan->writeformat == peer->readformat) {
 		/* Already compatible!  Moving on ... */
@@ -3812,8 +3813,9 @@ int ast_channel_make_compatible(struct ast_channel *chan, struct ast_channel *pe
 	 * no direct conversion available. If generic PLC is
 	 * desired, then transcoding via SLINEAR is a requirement
 	 */
+	use_slin = (src == AST_FORMAT_SLINEAR || dst == AST_FORMAT_SLINEAR);
 	if ((src != dst) && (ast_opt_generic_plc || ast_opt_transcode_via_slin) &&
-	    (ast_translate_path_steps(dst, src) != 1))
+	    (ast_translate_path_steps(dst, src) != 1 || use_slin))
 		dst = AST_FORMAT_SLINEAR;
 	if (ast_set_read_format(peer, dst) < 0) {
 		ast_log(LOG_WARNING, "Unable to set read format on channel %s to %d\n", peer->name, dst);