Fix memory leak in GeneProductAssociation::getFbcAssociationFor.

[luciansmith]

'addAssociation' cloned and added the object, but didn't delete it; now we call 'appendAndOwn' directly so a) we don't leak, and b) we're more efficient.

Still need to delete the object on failure.

[fbergmann]

while fbcand / fbcor ::addAssociation do not clone the association (they just call return mAssociations.append(fa); after some checks), in case of failure the added association should still be deleted.

[luciansmith]

'append' clones! That's why it's different from 'appendAndOwn'.

  /**
   * Adds an item to the end of this ListOf's list of items.
   *
   * This method makes a clone of the @p item handed to it.  This means that
   * when the ListOf object is destroyed, the original items will not be
   * destroyed.  For a method with an alternative ownership behavior, see the
   * ListOf::appendAndOwn(@if java SBase@endif) method.
   *
   * @param item the item to be added to the list.
   *
   * @copydetails doc_returns_success_code
   * @li @sbmlconstant{LIBSBML_OPERATION_SUCCESS, OperationReturnValues_t}
   * @li @sbmlconstant{LIBSBML_INVALID_OBJECT, OperationReturnValues_t}
   *
   * @see appendAndOwn(SBase* disownedItem)
   * @see appendFrom(const ListOf* list)
   */
  int append (const SBase* item);

Thanks for the review! Merging now.