Ensure crc32 checksum is checked only at the end of the reader

The caller can pass in an empty slice for reading, which should return
`Ok(0)`, not raise an error about an invalid checksum prematurely

src/crc32.rs

@@ -42,7 +42,7 @@ impl<R: Read> Read for Crc32Reader<R>
     {
         let count = match self.inner.read(buf)
         {
-            Ok(0) if !self.check_matches() => { return Err(io::Error::new(io::ErrorKind::Other, "Invalid checksum")) },
+            Ok(0) if !buf.is_empty() && !self.check_matches() => { return Err(io::Error::new(io::ErrorKind::Other, "Invalid checksum")) },
             Ok(n) => n,
             Err(e) => return Err(e),
         };
@@ -52,7 +52,7 @@ impl<R: Read> Read for Crc32Reader<R>
 }
 
 #[cfg(test)]
-mod tests
+mod test
 {
     use super::*;
     use std::io::Read;
@@ -86,4 +86,15 @@ mod tests
         // Can keep reading 0 bytes after the end
         assert_eq!(reader.read(&mut buf).unwrap(), 0);
     }
+
+    #[test]
+    fn test_zero_read()
+    {
+        let data: &[u8] = b"1234";
+        let mut buf = [0; 5];
+
+        let mut reader = Crc32Reader::new(data, 0x9be3e0a3);
+        assert_eq!(reader.read(&mut buf[..0]).unwrap(), 0);
+        assert_eq!(reader.read(&mut buf).unwrap(), 4);
+    }
 }