Change IndirectReferenceTable::Add null DCHECK to CHECK.

Change-Id: I7d3582bd5ef75c7fef6a94c0e4f1fea38fc8b526
diff --git a/runtime/indirect_reference_table.cc b/runtime/indirect_reference_table.cc
index 2bd8353..8194a0d 100644
--- a/runtime/indirect_reference_table.cc
+++ b/runtime/indirect_reference_table.cc
@@ -80,7 +80,7 @@
   prevState.all = cookie;
   size_t topIndex = segment_state_.parts.topIndex;
 
-  DCHECK(obj != NULL);
+  CHECK(obj != NULL);
   // TODO: stronger sanity check on the object (such as in heap)
   DCHECK_ALIGNED(reinterpret_cast<uintptr_t>(obj), 8);
   DCHECK(table_ != NULL);