(bug 31740) JpegMetadataExtractor and friends weren't checking for unexpected end...

(bug 31740) JpegMetadataExtractor and friends weren't checking for unexpected end of input properly

I created a new wrapper around unpack - wfUnpack which throws an exception if it runs out of input
(but i didn't use that on GIFMetadataExtractor/PNGMetadataExtractor because those files have a header saying that they weren't external dependencies minimized for potential re-users. I don't think anyone actually re-uses those files, but I didn't want to add a dependency on wfUnpack just in case).

I also changed fopen( blah, "r" ) -> fopen( blah, "rb" ) since these are binary formats, so we don't want newlines converted on windows.