Free attributes memory after tag creation

In plc_tag_create, a call to attr_create_from_str allocates memory
that is passed to ab_tag_create, but then no longer used or referenced.
Added attr_destroy call after the return of ab_tag_create to free
memory.

Also, removed if-statement as it does not appear to do anything.
Merge pull request #3 from ericorange/free_attributes

Free attributes memory after tag creation