Ensure selected page size works with allocate_string

Previously setting a large page size (i.e. 1M) would cause dynamic string
allocation to assert spuriously. A page size of 64K guarantees that all
offsets fit into 16 bits.

src/pugixml.cpp

@@ -400,6 +400,8 @@ PUGI__NS_BEGIN
 
 		char_t* allocate_string(size_t length)
 		{
+			PUGI__STATIC_ASSERT(xml_memory_page_size <= (1 << 16));
+
 			// allocate memory for string and header block
 			size_t size = sizeof(xml_memory_string_header) + length * sizeof(char_t);