Fixes unlocked read from logBuffer.LastTsNs that is racey. (#5536)

weed/util/log_buffer/log_read.go

@@ -66,9 +66,17 @@ func (logBuffer *LogBuffer) LoopProcessLogData(readerName string, startPosition
 				isDone = true
 				return
 			}
+			logBuffer.RLock()
 			lastTsNs := logBuffer.LastTsNs
-			for lastTsNs == logBuffer.LastTsNs {
+			logBuffer.RUnlock()
+			loopTsNs := lastTsNs // make a copy
+
+			for lastTsNs == loopTsNs {
 				if waitForDataFn() {
+					// Update loopTsNs and loop again
+					logBuffer.RLock()
+					loopTsNs = logBuffer.LastTsNs
+					logBuffer.RUnlock()
 					continue
 				} else {
 					isDone = true