Fix another may not be init compile error on AIX

AIX compiler is complaining that variable may not be initialized

	modified:   src/heap/heap.cc

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

BUG=

Review URL: https://codereview.chromium.org/1123743002

Cr-Commit-Position: refs/heads/master@{#28212}

src/heap/heap.cc

@@ -2416,7 +2416,7 @@ void Heap::ConfigureInitialOldGenerationSize() {
 
 AllocationResult Heap::AllocatePartialMap(InstanceType instance_type,
                                           int instance_size) {
-  Object* result;
+  Object* result = nullptr;
   AllocationResult allocation = AllocateRaw(Map::kSize, MAP_SPACE, MAP_SPACE);
   if (!allocation.To(&result)) return allocation;