[Coverity 1437923] Using invalid iterator in matroska_segment_seeker.cpp

Undefined behavior may result; the program may crash or subtly misbehave.

In mkv::​SegmentSeeker::​get_seekpoints_around(long, std::​vector<mkv::​SegmentSeeker::​Seekpoint, std::​allocatormkv::​SegmentSeeker::​Seekpoint> const &): An invalid or past-the-end iterator is being used (CWE-119)

222    iterator const it_begin  = seekpoints.begin();
      2. past_the_end: Function end creates an iterator.
      3. assign: Assigning: it_end = seekpoints->end().
223    iterator const it_end    = seekpoints.end();
224    iterator const it_middle = greatest_lower_bound( it_begin, it_end, needle );
225
      4. Condition it_middle != it_end, taking false branch.
      5. tested_end: it_middle testing equal to it_end.
226    if ( it_middle != it_end && (*it_middle).pts > pts)
227        // found nothing low enough, use the first one
228        return seekpoint_pair_t( *it_begin, Seekpoint() );
229
      6. assign: Assigning: it_before = it_middle.
230    iterator it_before = it_middle;
      7. assign: Assigning: <temporary> = it_middle.
      8. assign: Assigning: it_after = (it_middle == it_end) ? __gnu_cxx::__normal_iterator<mkv::SegmentSeeker::Seekpoint const *, std::vector<mkv::SegmentSeeker::Seekpoint, std::allocator<mkv::SegmentSeeker::Seekpoint> > >(it_middle) : mkv::SegmentSeeker::get_seekpoints_around(long, std::vector<mkv::SegmentSeeker::Seekpoint, std::allocator<mkv::SegmentSeeker::Seekpoint> > const &)::iterator const(<unnamed>::next_(it_middle)).
231    iterator it_after = it_middle == it_end ? it_middle : next_( it_middle ) ;
232
     
CID 1437923: (#1 of 1): Using invalid iterator (INVALIDATE_ITERATOR)
9. deref_iterator: Dereferencing iterator it_before though it is already past the end of its container.
233    return seekpoint_pair_t( *it_before,
234      it_after == it_end ? Seekpoint() : *it_after
235    );