app_voicemail: Disable ADSI if unavailable.

If ADSI is available on a channel, app_voicemail will repeatedly
try to use ADSI, even if there is no CPE that supports it. This
leads to many unnecessary delays during the session. If ADSI is
available but ADSI setup fails, we now disable it to prevent
further attempts to use ADSI during the session.

Resolves: #354

apps/app_voicemail.c

@@ -7451,8 +7451,11 @@ static void adsi_begin(struct ast_channel *chan, int *useadsi)
 	if (!ast_adsi_available(chan))
 		return;
 	x = ast_adsi_load_session(chan, adsifdn, adsiver, 1);
-	if (x < 0)
+	if (x < 0) {
+		*useadsi = 0;
+		ast_channel_adsicpe_set(chan, AST_ADSI_UNAVAILABLE);
 		return;
+	}
 	if (!x) {
 		if (adsi_load_vmail(chan, useadsi)) {
 			ast_log(AST_LOG_WARNING, "Unable to upload voicemail scripts\n");