Avoid crash in HeapChunkContext::Flush post Reset.

Port to ART of 22d8dba8ae5be7aecb07322a410712206896fa15.

Change-Id: I74445f2aac22a037533f13aa894ae4f6cea1abc9
diff --git a/src/debugger.cc b/src/debugger.cc
index 49c29ba..cdc2178 100644
--- a/src/debugger.cc
+++ b/src/debugger.cc
@@ -3182,6 +3182,11 @@
   }
 
   void Flush() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    if (pieceLenField_ == NULL) {
+      // Flush immediately post Reset (maybe back-to-back Flush). Ignore.
+      CHECK(needHeader_);
+      return;
+    }
     // Patch the "length of piece" field.
     CHECK_LE(&buf_[0], pieceLenField_);
     CHECK_LE(pieceLenField_, p_);