Avoid libc++ assert failure when building with _LIBCPP_DEBUG=0

libc++ will assert when indexing one element past the end of a vector, but V8
uses this as the end iterator for ScopedPtrList.  Similarly, when there's no
elements in the vector, v[0] will also assert, so ScopedPtrList::begin() needs
to be updated too.  This CL changes ScopedPtrList to use std::vector::data() to
get the iterators.

BUG=chromium:923166
TBR=machenbach

Change-Id: Ic6a5176611d52ed592da743ecce44287c452b379
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1565543
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60851}

src/zone/zone.h

@@ -390,9 +390,11 @@ class ScopedPtrList final {
 
   typedef T** iterator;
   inline iterator begin() const {
-    return reinterpret_cast<T**>(&buffer_[start_]);
+    return reinterpret_cast<T**>(buffer_.data() + start_);
+  }
+  inline iterator end() const {
+    return reinterpret_cast<T**>(buffer_.data() + end_);
   }
-  inline iterator end() const { return reinterpret_cast<T**>(&buffer_[end_]); }
 
  private:
   std::vector<void*>& buffer_;