Fix sudden nil pointer dereference in walk

lib/scanner/walk.go

@@ -240,7 +240,7 @@ func (w *Walker) walkAndHashFiles(fchan, dchan chan protocol.FileInfo) filepath.
 		}
 
 		if sn := filepath.Base(rn); sn == ".stignore" || sn == ".stfolder" ||
-			strings.HasPrefix(rn, ".stversions") || w.Matcher.Match(rn) {
+			strings.HasPrefix(rn, ".stversions") || (w.Matcher != nil && w.Matcher.Match(rn)) {
 			// An ignored file
 			if debug {
 				l.Debugln("ignored:", rn)