lib/scanner: Recheck file size and modification time after hashing (ref #3440)

To catch the case where the file changed. Also make sure we never let a
size-vs-blocklist mismatch slip through.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3443

lib/scanner/blocks.go

@@ -29,7 +29,7 @@ func Blocks(r io.Reader, blocksize int, sizehint int64, counter Counter) ([]prot
 	var blocks []protocol.BlockInfo
 	var hashes, thisHash []byte
 
-	if sizehint > 0 {
+	if sizehint >= 0 {
 		// Allocate contiguous blocks for the BlockInfo structures and their
 		// hashes once and for all, and stick to the specified size.
 		r = io.LimitReader(r, sizehint)