More ast_strlen_zero changes

apps/app_directory.c

apps/app_enumlookup.c

apps/app_festival.c

apps/app_groupcount.c

apps/app_playback.c

apps/app_queue.c

apps/app_read.c

apps/app_senddtmf.c

apps/app_setcdruserfield.c

apps/app_setcidname.c

apps/app_setcidnum.c

channels/chan_agent.c

frame.c