Avoid exit-time destructors.

operator[]() was recently changed to use the existing code in order to
correctly align the returned pointer; however this broke
-Wexit-time-destructors.  Change to a method that is still correctly
aligned but does not generate a destructor.

include/rapidjson/document.h

@@ -1235,8 +1235,8 @@ public:
             // return NullValue;
 
             // Use static buffer and placement-new to prevent destruction
-            static GenericValue buffer;
+            alignas(GenericValue) static char buffer[sizeof(GenericValue)];
-            return *new (reinterpret_cast<char *>(&buffer)) GenericValue();
+            return *new (buffer) GenericValue();
         }
     }
     template <typename SourceAllocator>