PartitionAlloc: PCScan: Add missing null check.

If maybe_ptr is within super-page payload, but refers to not allocated
memory, null is returned from GetSlotStartInSuperPage. The check for
returned null was missing.

Bug: 11297512
Change-Id: I4284b1c79f353c32c96e9cbc75d0200aa452bf30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2549921
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829256}

base/allocator/partition_allocator/pcscan.cc

@@ -205,7 +205,7 @@ size_t PCScan<thread_safe>::PCScanTask::TryMarkObjectInNormalBucketPool(
   // Check if pointer was in the quarantine bitmap.
   const uintptr_t base =
       GetObjectStartInSuperPage<thread_safe>(maybe_ptr, root_);
-  if (!bitmap->CheckBit(base))
+  if (!base || !bitmap->CheckBit(base))
     return 0;
 
   PA_DCHECK((maybe_ptr & kSuperPageBaseMask) == (base & kSuperPageBaseMask));