Use ast_strlen_zero to avoid a crash when a Dial() string isn't passed to ParkAndAnnounce

(closes issue #16731)
Reported by: sebele67
Patches:
      issue16731_20100129.diff uploaded by seanbright (license 71)
Tested by: sebele67


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@251410 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_parkandannounce.c

@@ -112,7 +112,7 @@ static int parkandannounce_exec(struct ast_channel *chan, void *data)
 		timeout *= 1000;
 	}
 	dial = strsep(&s, "|");
-	if(!dial) {
+	if (ast_strlen_zero(dial)) {
 		ast_log(LOG_WARNING, "PARK: A dial resource must be specified i.e: Console/dsp or Zap/g1/5551212\n");
 		ast_module_user_remove(u);
 		return -1;