Fix segv in allocation tracking.

Change-Id: I19b729c94b8037b16a220af2213e8d4bfa92e04d
diff --git a/src/debugger.cc b/src/debugger.cc
index 342d548..e928b46 100644
--- a/src/debugger.cc
+++ b/src/debugger.cc
@@ -3049,8 +3049,8 @@
     MethodHelper mh;
     for (size_t i = 0; i < kMaxAllocRecordStackDepth; i++) {
       Method* m = record->stack[i].method;
-      mh.ChangeMethod(m);
       if (m != NULL) {
+        mh.ChangeMethod(m);
         class_names.Add(mh.GetDeclaringClassDescriptor());
         method_names.Add(mh.GetName());
         filenames.Add(mh.GetDeclaringClassSourceFile());