Null Pointer check.

ref = (WBXMLStringTableElement *) wbxml_list_get(referenced, j);
            
In reference to  : https://github.com/libwbxml/libwbxml/commit/9f7fb3008fc0156e8bfd12c8e4cb41c70e315c68

wbxml_get_list can return NULL. Hence good to do Null check before dereferencing.
Merge pull request #4 from ya1gaurav/patch-4

Null Pointer check.