array: always return non-NULL pointer from ARR_GetElements()

Some libc calls like memcpy() expect the pointer to be valid even when
the size is zero and there is nothing to do. Instead of checking the
size before all such calls, modify ARR_GetElements() to return a pointer
to the array instance itself if data was not allocated yet.

array.c

@@ -103,6 +103,12 @@ ARR_GetElement(ARR_Instance array, unsigned int index)
 void *
 ARR_GetElements(ARR_Instance array)
 {
+  /* Return a non-NULL pointer when the array has zero size */
+  if (!array->data) {
+    assert(!array->used);
+    return array;
+  }
+
   return array->data;
 }