Add an assert to ensure we never allocate second empty fixed array.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 25d199b189d5e881797fbf829495b4c5790c017d..6d46740b2ef7b9955261c42978f31dcd215ad3af 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -3195,6 +3195,7 @@ Object* Heap::AllocateRawFixedArray(int length) {
   if (length < 0 || length > FixedArray::kMaxLength) {
     return Failure::OutOfMemoryException();
   }
+  ASSERT(length > 0);
   // Use the general function if we're forced to always allocate.
   if (always_allocate()) return AllocateFixedArray(length, TENURED);
   // Allocate the raw data for a fixed array.