freetdm: check if the event was created before firing it

libs/freetdm/mod_freetdm/mod_freetdm.c

@@ -1855,6 +1855,7 @@ static FIO_SIGNAL_CB_FUNCTION(on_common_signal)
 		break;
 	}
 
+	if (event) {
 		switch_event_add_header(event, SWITCH_STACK_BOTTOM, "span-name", "%s", ftdm_channel_get_span_name(sigmsg->channel));
 		switch_event_add_header(event, SWITCH_STACK_BOTTOM, "span-number", "%d", ftdm_channel_get_span_id(sigmsg->channel));
 		switch_event_add_header(event, SWITCH_STACK_BOTTOM, "chan-number", "%d", ftdm_channel_get_id(sigmsg->channel));
@@ -1878,6 +1879,7 @@ static FIO_SIGNAL_CB_FUNCTION(on_common_signal)
 			switch_event_add_header_string(event, SWITCH_STACK_BOTTOM, "alarm", "general");
 		}
 		switch_event_fire(&event);
+	}
 
 	return FTDM_BREAK;
 }