previously, specifying -hls_segment_filename meant
s->base_output_dirname was never set, causing a segfault:

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
* frame #0: 0x00007fffbf7d82d0 libsystem_platform.dylib`_platform_strcmp + 80
frame #1: 0x00000001005f4dcf libavformat.57.dylib`io_open_default(s=0x000000010481cc00, pb=0x0000000104806020, url=0x0000000000000000, flags=2, options=0x0000000000000000) at options.c:107
frame #2: 0x0000000100528968 libavformat.57.dylib`hls_mux_init(s=0x000000010481cc00) at hlsenc.c:595
frame #3: 0x00000001005273cb libavformat.57.dylib`hls_write_header(s=0x000000010481cc00) at hlsenc.c:1518
frame #4: 0x00000001005c08d0 libavformat.57.dylib`write_header_internal(s=0x000000010481cc00) at mux.c:486
frame #5: 0x00000001005c0774 libavformat.57.dylib`avformat_write_header(s=0x000000010481cc00, options=0x00000001029026e8) at mux.c:539