fix crash when we populate Exif tags from Pentax makernotes ShutterCount.

This only occurs with Exiv2 0.26, not 0.25.
Change to use operator
   std::ostringstream << &Exiv2::ExifData::const_iterator
by
   Exiv2::ExifData::const_iterator->write(std::ostringstream, &Exiv2::ExifData)
as Exiv2::actions.cpp do to print Exif tags values.
Exerywhere, use Exiv2 metadata container const iterator when we use iterator in read only.
BUGS: 374922
FIXED-IN: 5.5.0