Use ResetTlab instead of setting to null

Added a function ResetTlab to reset the TLAB to empty. This is
in case the empty TLAB doesn't have null values.

Bug: 139805154
Test: make
Change-Id: Ife2225fc534999e53f2ecad41fc29e46c90a8817
diff --git a/runtime/thread.h b/runtime/thread.h
index 0f6b369..32a620c 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -1182,6 +1182,7 @@
   mirror::Object* AllocTlab(size_t bytes);
   void SetTlab(uint8_t* start, uint8_t* end, uint8_t* limit);
   bool HasTlab() const;
+  void ResetTlab();
   uint8_t* GetTlabStart() {
     return tlsPtr_.thread_local_start;
   }