Implement review comment

Author: GoeLin

src/hotspot/share/gc/g1/g1CollectedHeap.cpp

@@ -419,6 +419,8 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size, bool allow_
         result = _allocator->attempt_allocation_using_new_region(word_size);
         if (result != NULL) {
           return result;
+        } else if (!allow_gc) {
+          return nullptr;
         }
 
         // If the GCLocker is active and we are bound for a GC, try expanding young gen.
@@ -452,8 +454,6 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(size_t word_size, bool allow_
         log_trace(gc, alloc)("%s: Successfully scheduled collection returning " PTR_FORMAT,
                              Thread::current()->name(), p2i(result));
         return result;
-      } else if (!allow_gc) {
-        return nullptr;
       }
 
       if (succeeded) {