AIX: Fix 'may be used uninitialized' compiler errors

Fix additional cases where the AIX compiler reports that a variable
may be used uninitialized.

R=danno@chromium.org, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1323313003

Cr-Commit-Position: refs/heads/master@{#30658}

src/heap/heap-inl.h

@@ -126,7 +126,7 @@ AllocationResult Heap::AllocateOneByteInternalizedString(
   AllocationSpace space = SelectSpace(size, TENURED);
 
   // Allocate string.
-  HeapObject* result;
+  HeapObject* result = nullptr;
   {
     AllocationResult allocation = AllocateRaw(size, space, OLD_SPACE);
     if (!allocation.To(&result)) return allocation;
@@ -158,7 +158,7 @@ AllocationResult Heap::AllocateTwoByteInternalizedString(Vector<const uc16> str,
   AllocationSpace space = SelectSpace(size, TENURED);
 
   // Allocate string.
-  HeapObject* result;
+  HeapObject* result = nullptr;
   {
     AllocationResult allocation = AllocateRaw(size, space, OLD_SPACE);
     if (!allocation.To(&result)) return allocation;
@@ -206,7 +206,7 @@ AllocationResult Heap::AllocateRaw(int size_in_bytes, AllocationSpace space,
   isolate_->counters()->objs_since_last_young()->Increment();
 #endif
 
-  HeapObject* object;
+  HeapObject* object = nullptr;
   AllocationResult allocation;
   if (NEW_SPACE == space) {
     allocation = new_space_.AllocateRaw(size_in_bytes, alignment);

src/heap/mark-compact.cc

@@ -2941,7 +2941,7 @@ bool MarkCompactCollector::TryPromoteObject(HeapObject* object,
 
   OldSpace* old_space = heap()->old_space();
 
-  HeapObject* target;
+  HeapObject* target = nullptr;
   AllocationAlignment alignment = object->RequiredAlignment();
   AllocationResult allocation = old_space->AllocateRaw(object_size, alignment);
   if (allocation.To(&target)) {
@@ -3193,7 +3193,7 @@ void MarkCompactCollector::EvacuateLiveObjectsFromPage(Page* p) {
 
       int size = object->Size();
       AllocationAlignment alignment = object->RequiredAlignment();
-      HeapObject* target_object;
+      HeapObject* target_object = nullptr;
       AllocationResult allocation = space->AllocateRaw(size, alignment);
       if (!allocation.To(&target_object)) {
         // If allocation failed, use emergency memory and re-try allocation.