Avoid a crash:

https://github.com/syncthing/syncthing/issues/5002

Patch copied from upstream source repository:

https://github.com/syncthing/syncthing/commit/35a75a95dc6383b2d73ab645f1407f7907ec1a2c

From 35a75a95dc6383b2d73ab645f1407f7907ec1a2c Mon Sep 17 00:00:00 2001
From: Jakob Borg <jakob@kastelo.net>
Date: Wed, 13 Jun 2018 19:07:52 +0200
Subject: [PATCH] lib/model: Don't panic when rechecking file (fixes #5002)
 (#5003)

---
 lib/model/model.go      |  2 +-
 lib/model/model_test.go | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/lib/model/model.go b/lib/model/model.go
index 5a9146e0..302f06c5 100644
--- a/lib/model/model.go
+++ b/lib/model/model.go
@@ -1373,7 +1373,7 @@ func (m *Model) recheckFile(deviceID protocol.DeviceID, folderFs fs.Filesystem,
 		return
 	}
 
-	if blockIndex > len(cf.Blocks) {
+	if blockIndex >= len(cf.Blocks) {
 		l.Debugf("%v recheckFile: %s: %q / %q i=%d: block index too far", m, deviceID, folder, name, blockIndex)
 		return
 	}
diff --git a/lib/model/model_test.go b/lib/model/model_test.go
index 295eafc1..456bbc4a 100644
--- a/lib/model/model_test.go
+++ b/lib/model/model_test.go
@@ -3608,6 +3608,32 @@ func TestIssue4903(t *testing.T) {
 	}
 }
 
+func TestIssue5002(t *testing.T) {
+	// recheckFile should not panic when given an index equal to the number of blocks
+
+	db := db.OpenMemory()
+	m := NewModel(defaultCfgWrapper, protocol.LocalDeviceID, "syncthing", "dev", db, nil)
+	m.AddFolder(defaultFolderConfig)
+	m.StartFolder("default")
+
+	m.ServeBackground()
+	defer m.Stop()
+
+	if err := m.ScanFolder("default"); err != nil {
+		t.Error(err)
+	}
+
+	file, ok := m.CurrentFolderFile("default", "foo")
+	if !ok {
+		t.Fatal("test file should exist")
+	}
+	nBlocks := len(file.Blocks)
+
+	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", nBlocks-1, []byte{1, 2, 3, 4})
+	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", nBlocks, []byte{1, 2, 3, 4}) // panic
+	m.recheckFile(protocol.LocalDeviceID, defaultFolderConfig.Filesystem(), "default", "foo", nBlocks+1, []byte{1, 2, 3, 4})
+}
+
 func addFakeConn(m *Model, dev protocol.DeviceID) *fakeConnection {
 	fc := &fakeConnection{id: dev, model: m}
 	m.AddConnection(fc, protocol.HelloResult{})
-- 
2.18.0