avformat/dashdec: check init_section before use it.

avformat/dashdec: check init_section before use it.

because there have no Initialization in SegmentTemplate,
so it will have no init_section for init segment file.
but in the is_common_init_section_exist function it will be used for
check to url, url_offset and size, so check init_section
before use init_section.
And fix code style in is_common_init_section_exist,
make the code block short when it too long.

fix ticket: 9062

Signed-off-by: liuqi05 <liuqi05@kuaishou.com>