deps: v8: fix potential segfault in profiler

This change fixes a potential segfault in the sampling heap profiler.
This landed as part of a larger change upstream [1]. This is the minimal
backport that avoids the segfault.

[1]: https://git.io/vdTYL

PR-URL: https://github.com/nodejs/node/pull/15498
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

deps/v8/src/profiler/sampling-heap-profiler.h

@@ -172,8 +172,11 @@ class SamplingAllocationObserver : public AllocationObserver {
   void Step(int bytes_allocated, Address soon_object, size_t size) override {
     USE(heap_);
     DCHECK(heap_->gc_state() == Heap::NOT_IN_GC);
-    DCHECK(soon_object);
-    profiler_->SampleObject(soon_object, size);
+    if (soon_object) {
+      // TODO(ofrobots): it would be better to sample the next object rather
+      // than skipping this sample epoch if soon_object happens to be null.
+      profiler_->SampleObject(soon_object, size);
+    }
   }
 
   intptr_t GetNextStepSize() override { return GetNextSampleInterval(rate_); }