check value before using. Found by Klockwork (www.klocwork.com)

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@8447 d0543943-73ff-0310-b7d9-9358b9ac24b2

src/switch_ivr.c

@@ -577,7 +577,7 @@ SWITCH_DECLARE(switch_status_t) switch_ivr_collect_digits_callback(switch_core_s
 	switch_time_t started = 0;
 	uint32_t elapsed;
 
-	if (!args->input_callback) {
+	if (!args || !args->input_callback) {
 		return SWITCH_STATUS_GENERR;
 	}