Mp4: fixed non-keyframe seeks in some cases (ticket #175). Number of entries in stsc atom was wrong if we've added an entry to split a chunk. Additionally, there is no need to add an entry if we are going to split last chunk in an entry, it's enough to update the entry we already have. Previously new entry was added and old one was left as is, resulting in incorrect entry with zero chunks which might confuse some software.

# Copyright (C) Igor Sysoev
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have=NGX_SUPPRESS_WARN . auto/have

have=NGX_SMP . auto/have