Fix compile failure for AIX

Fix another may be uninitialized compile failure on AIX
in src/heap/heap.cc

R=svenpanne@chromium.org, mbrandy@us.ibm.com

BUG=

Review URL: https://codereview.chromium.org/1153233004

Cr-Commit-Position: refs/heads/master@{#28710}

src/heap/heap.cc

@@ -4481,7 +4481,7 @@ AllocationResult Heap::AllocateConstantPoolArray(
   int size = ConstantPoolArray::SizeFor(small);
   AllocationSpace space = SelectSpace(size, TENURED);
 
-  HeapObject* object;
+  HeapObject* object = nullptr;
   {
     AllocationResult allocation =
         AllocateRaw(size, space, OLD_SPACE, kDoubleAligned);