null check stat when we check file on disk

https://sentry.io/sharelatex-1/sl-web-server-prod/issues/125814174/

services/web/app/coffee/Features/FileStore/FileStoreHandler.coffee

@@ -13,6 +13,9 @@ module.exports = FileStoreHandler =
 			if err?
 				logger.err err:err, project_id:project_id, file_id:file_id, fsPath:fsPath, "error stating file"
 				callback(err)
+			if !stat?
+				logger.err project_id:project_id, file_id:file_id, fsPath:fsPath, "stat is not available, can not check file from disk"
+				return callback(new Error("error getting stat, not available"))
 			if !stat.isFile()
 				logger.log project_id:project_id, file_id:file_id, fsPath:fsPath, "tried to upload symlink, not contining"
 				return callback(new Error("can not upload symlink"))

services/web/test/UnitTests/coffee/FileStore/FileStoreHandlerTests.coffee

@@ -96,6 +96,13 @@ describe "FileStoreHandler", ->
 					@fs.createReadStream.called.should.equal false
 					done()
 
+		describe "symlink", ->
+			it "should not read file stat returns nothing", (done)->
+				@fs.lstat = sinon.stub().callsArgWith(1, null, null)
+				@handler.uploadFileFromDisk @project_id, @file_id, @fsPath, =>
+					@fs.createReadStream.called.should.equal false
+					done()
+
 		describe "when upload fails", ->
 			beforeEach ->
 				@writeStream.on = (type, cb) ->