Add array bounds assertion and improve type conversion for empty strings.

mojo/public/bindings/lib/bindings_internal.h

Index: mojo/public/bindings/lib/bindings_internal.h
diff --git a/mojo/public/bindings/lib/bindings_internal.h b/mojo/public/bindings/lib/bindings_internal.h
index 1d5a24a1e65352e6823aaa2bcf0776a8921bec25..c78db05830bd9a4820db46682cd84a92b3d4dc33 100644
--- a/mojo/public/bindings/lib/bindings_internal.h
+++ b/mojo/public/bindings/lib/bindings_internal.h
@@ -163,10 +163,12 @@ class Array_Data {
   size_t size() const { return header_.num_elements; }
 
   T& at(size_t offset) {
+    assert(offset < static_cast<size_t>(header_.num_elements));
     return Traits::ToRef(storage()[offset]);
   }
 
   const T& at(size_t offset) const {
+    assert(offset < static_cast<size_t>(header_.num_elements));
     return Traits::ToConstRef(storage()[offset]);
   }