PartitionAlloc: Add some checks to try to help debug issue 705605.

Bug: 705605
Change-Id: I7a056679917692e3ce326ac1553b2be110c9fdc5
Reviewed-on: https://chromium-review.googlesource.com/1239159Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594043}

base/allocator/partition_allocator/partition_alloc.h

@@ -337,6 +337,7 @@ ALWAYS_INLINE internal::PartitionBucket* PartitionGenericSizeToBucket(
   internal::PartitionBucket* bucket =
       root->bucket_lookups[(order << kGenericNumBucketsPerOrderBits) +
                            order_index + !!sub_order_index];
+  CHECK(bucket);
   DCHECK(!bucket->slot_size || bucket->slot_size >= size);
   DCHECK(!(bucket->slot_size % kGenericSmallestBucket));
   return bucket;