Fix ASAN failure in `array.rend()`. (#219)

The iterator constructor used `array[index]` to initialize an internal
cached view (necessary so that `operator->` can return a pointer), which
caused pointer arithmetic overflow when `index` was -1 -- as in the case
of `rend()`.

This change uses a checked `array.at(index)` method to get a null view
when `index` is out of bounds.

I considered changing `array[index]` to return a null view for
out-of-bounds indexes, but user code may be inadvertently relying on
that behavior, so I am leaving it out of this bug fix.

runtime/cpp/emboss_array_view.h

diff --git a/runtime/cpp/emboss_array_view.h b/runtime/cpp/emboss_array_view.h
index 674b2ad..fa8ccd3 100644
--- a/runtime/cpp/emboss_array_view.h
+++ b/runtime/cpp/emboss_array_view.h
@@ -56,7 +56,7 @@
 
   explicit ElementViewIterator(const GenericArrayView array_view,
                                ::std::ptrdiff_t index)
-      : array_view_(array_view), view_(array_view[index]), index_(index) {}
+      : array_view_(array_view), view_(array_view.at(index)), index_(index) {}
 
   ElementViewIterator() = default;
 
@@ -66,7 +66,7 @@
 
   ElementViewIterator &operator+=(difference_type d) {
     index_ += (kDirection == ElementViewIteratorDirection::kForward ? d : -d);
-    view_ = array_view_[index_];
+    view_ = array_view_.at(index_);
     return *this;
   }
 
@@ -178,9 +178,15 @@
       : parameters_{parameters...}, buffer_{buffer} {}
 
   ElementView operator[](::std::size_t index) const {
-    return IndexOperatorHelper<sizeof...(ElementViewParameterTypes) ==
-                               0>::ConstructElement(parameters_, buffer_,
-                                                    index);
+    return IndexOperatorHelper<(sizeof...(ElementViewParameterTypes) ==
+                                0)>::UncheckedConstructElement(parameters_,
+                                                               buffer_, index);
+  }
+
+  ElementView at(::std::size_t index) const {
+    return IndexOperatorHelper<(sizeof...(ElementViewParameterTypes) ==
+                                0)>::ConstructElement(parameters_, buffer_,
+                                                      index, ElementCount());
   }
 
   ForwardIterator begin() const { return ForwardIterator(*this, 0); }
@@ -316,10 +322,18 @@
   struct IndexOperatorHelper {
     static ElementView ConstructElement(
         const ::std::tuple<ElementViewParameterTypes...> &parameters,
+        BufferType buffer, ::std::size_t index, ::std::size_t size) {
+      return IndexOperatorHelper<
+          (sizeof...(ElementViewParameterTypes) == 1 + sizeof...(N)), N...,
+          sizeof...(N)>::ConstructElement(parameters, buffer, index, size);
+    }
+
+    static ElementView UncheckedConstructElement(
+        const ::std::tuple<ElementViewParameterTypes...> &parameters,
         BufferType buffer, ::std::size_t index) {
       return IndexOperatorHelper<
-          sizeof...(ElementViewParameterTypes) == 1 + sizeof...(N), N...,
-          sizeof...(N)>::ConstructElement(parameters, buffer, index);
+          (sizeof...(ElementViewParameterTypes) == 1 + sizeof...(N)), N...,
+          sizeof...(N)>::UncheckedConstructElement(parameters, buffer, index);
     }
   };
 
@@ -327,6 +341,18 @@
   struct IndexOperatorHelper<true, N...> {
     static ElementView ConstructElement(
         const ::std::tuple<ElementViewParameterTypes...> &parameters,
+        BufferType buffer, ::std::size_t index, ::std::size_t size) {
+      return ElementView(
+          ::std::get<N>(parameters)...,
+          index < 0 || index >= size
+              ? typename BufferType::template OffsetStorageType<kElementSize,
+                                                                0>(nullptr)
+              : buffer.template GetOffsetStorage<kElementSize, 0>(
+                    kElementSize * index, kElementSize));
+    }
+
+    static ElementView UncheckedConstructElement(
+        const ::std::tuple<ElementViewParameterTypes...> &parameters,
         BufferType buffer, ::std::size_t index) {
       return ElementView(::std::get<N>(parameters)...,
                          buffer.template GetOffsetStorage<kElementSize, 0>(