exiv2: Fix use-after-free and double-free with exiv2 0.28.x

exiv2: Fix use-after-free and double-free with exiv2 0.28.x

In exiv2 0.28.x, DataBuf is only a wrapper for std::vector.
Exiv2::DataBuf::data() returns a shallow copy of its underlying
data.  On Exiv2::DataBuf::reset(), all of its data is invalidated,
includes the data returned by Exiv2::DataBuf::data(). Thus, all usages
of said data is use-after-free, attempt to use its later is double-free.

Make a copy ourselves instead.  This method works for the old exiv2, too.
Since Exiv2::PreviewImage::size() is available from forever, and both
Exiv2::PreviewImage::copy() and Exiv2::PreviewImage::pData() are
introduced in a same change (commit 9578e8123 in 2008).

Fixes: #1201
Fixes: https://github.com/void-linux/void-packages/issues/47547