CompressingReader: account for possible out buffer state

It is possible for out buffer to have no overflow data and yet
the read offset will be not zero (if byte slices passed by the reader
just happened to align perfectly).

Signed-off-by: Alex Dubov <[email protected]>

Author: oakad

compressing_reader.go

@@ -206,6 +206,11 @@ func (wr *ovWriter) reset(out []byte) bool {
 		wr.dataPos = ovRem
 	}
 
+	if wr.ovPos > 0 {
+		wr.ov = wr.ov[ : 0]
+		wr.ovPos = 0
+	}
+
 	wr.data = out
 	return true
 }