Fix read in packbits decoder

This assumed that a `read` would succeed for the full length, thus
interpreting in a subsequent call some data bits as a header.

For instance, consider 77 bytes of literal were left and a buffer of
1024 bytes was provided. Then the `read` call might return 60 which was
ignored and assumed to be 77. Thus the next 17 bytes of literal data
were instead interpreted as a header due to setting `self.count` to 0.

src/decoder/stream.rs

@@ -384,23 +384,25 @@ impl<R: Read> Read for PackBitsReader<R> {
         }
 
         let length = buf.len().min(self.count);
-        match self.state {
+        let actual = match self.state {
             PackBitsReaderState::Literal => {
-                self.reader.read(&mut buf[..length])?;
+                self.reader.read(&mut buf[..length])?
             }
             PackBitsReaderState::Repeat { value } => {
                 for b in &mut buf[..length] {
                     *b = value;
                 }
+
+                length
             }
             PackBitsReaderState::Header => unreachable!(),
-        }
+        };
 
-        self.count -= length;
+        self.count -= actual;
         if self.count == 0 {
             self.state = PackBitsReaderState::Header;
         }
-        return Ok(length);
+        return Ok(actual);
     }
 }