Fix multi-threaded access to logs

runtime/vm/log.cc

Index: runtime/vm/log.cc
diff --git a/runtime/vm/log.cc b/runtime/vm/log.cc
index b7f65891634e1e78fea1fc654ba4f770fb23d953..91be51d604faf0c502a899e967d8291053642a52 100644
--- a/runtime/vm/log.cc
+++ b/runtime/vm/log.cc
@@ -141,16 +141,26 @@ Log* Log::NoOpLog() {
 
 
 void Log::TerminateString() {
+  if (this == NoOpLog()) {
+    return;
+  }
   buffer_.Add('\0');
 }
 
 
 void Log::EnableManualFlush() {
+  if (this == NoOpLog()) {
+    return;
+  }
   manual_flush_++;
 }
 
 
 void Log::DisableManualFlush() {
+  if (this == NoOpLog()) {
+    return;
+  }
+
   manual_flush_--;
   ASSERT(manual_flush_ >= 0);
   if (manual_flush_ == 0) {