efl_ui_widget_common: Fix potential resource leak

efl_ui_widget_common: Fix potential resource leak

Coverity reports that we potentially leak the storage of
'tree_iterator' here because we allocate the space for it, but
potentially juse return due to use of ELM_WIDGET_DATA_GET_OR_RETURN.
To fix this, just move the allocation to after the above macro is
called.

Fixes Coverity CID1399088

@fix

Reviewed-by: Stefan Schmidt <stefan@datenfreihafen.org>
Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D8314