Update allocator_try_assign() to match behavior of fstream allocator for non-copyable types

Looks like the choice we have made here previously is to return NULL if
attempting to allocate types that we cannot allocate.

library/include/DataDefs.h

@@ -499,23 +499,27 @@ namespace df
 
     template<class T>
     typename std::enable_if<
-        std::is_copy_assignable<T>::value
+        std::is_copy_assignable<T>::value,
+        void*
     >::type allocator_try_assign(void *out, const void *in) {
         *(T*)out = *(const T*)in;
+        return out;
     }
 
     template<class T>
     typename std::enable_if<
-        !std::is_copy_assignable<T>::value
+        !std::is_copy_assignable<T>::value,
+        void*
     >::type allocator_try_assign(void *out, const void *in) {
         // assignment is not possible; do nothing
+        return NULL;
     }
 
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdelete-non-virtual-dtor"
     template<class T>
     void *allocator_fn(void *out, const void *in) {
-        if (out) { allocator_try_assign<T>(out, in); return out; }
+        if (out) { return allocator_try_assign<T>(out, in); }
         else if (in) { delete (T*)in; return (T*)in; }
         else return new T();
     }