Add a test for ZIP64 with leading junk.

I constructed this file using a hack from the Zip manpage: if the input to a
Zip compression command is streamed on standard input, the output is given in
ZIP64 format since the tool doesn't know how big the input will be. I modified
the resulting file by adding some leading junk text and editing the non-ZIP64
end-of-central-directory structure to have 0xFFFF for its "number of files"
parameters, to help the test demonstrate that the ZIP64 data are being
properly read. (0xFFFF is the value used in the non-ZIP64 structure if the
archive actually has more than 65535 files.)

src/read.rs

@@ -465,4 +465,15 @@ mod test {
         let reader = ZipArchive::new(io::Cursor::new(v));
         assert!(reader.is_err());
     }
+
+    #[test]
+    fn zip64_with_leading_junk() {
+        use std::io;
+        use super::ZipArchive;
+
+        let mut v = Vec::new();
+        v.extend_from_slice(include_bytes!("../tests/data/zip64_demo.zip"));
+        let reader = ZipArchive::new(io::Cursor::new(v)).unwrap();
+        assert!(reader.len() == 1);
+    }
 }