...as emitted by at least GCC 8.2 with --enable-optimized:

> In file included from vcl/source/gdi/pngread.cxx:27:
> include/vcl/pngread.hxx: In member function ‘bool vcl::PNGReaderImpl::ReadNextChunk()’:
> include/vcl/pngread.hxx:49:12: error: ‘aDummyChunk.vcl::PNGReader::ChunkData::nType’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      struct ChunkData
>             ^~~~~~~~~

The immediately relevant part is initializing

        sal_uInt32 nType = 0;

in the ChunkData ctor, but while at it, also make the Read*Int32 calls in
PNGReaderImpl::ReadNextChunk more robust.

Change-Id: Id6ad10a4382a8d063fd70c7bac595c3693475ca3
Reviewed-on: https://gerrit.libreoffice.org/66619
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>