avformat/hlsenc: Duplicated segment filenames and use_localtime_mkdir

Current implementation of finding duplicate segment filenames may fail
if use_localtime_mkdir and use_localtime are in effect and
segment_filename option expression contains subdirectories with
date/time specifiers. This patch fixes this false behaviour.

Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>