Fix Null Pointer Dereference.

In reference to : https://github.com/libwbxml/libwbxml/commit/9f7fb3008fc0156e8bfd12c8e4cb41c70e315c68
wbxml_list_get can return NULL.
returning in case elt is NULL.
Merge pull request #10 from ya1gaurav/patch-10

Fix Null Pointer Dereference.