edje_externals: Fix wrong free.
edje_externals: Fix wrong free.

_signal is used after free. _signal will be freed soon any way.
This was introduced in 3da3082.

This fixes coverity CID 1308263.

rvandegrift/elementary,tasn/elementary,FlorentRevest/Elementary,tasn/elementary,FlorentRevest/Elementary,rvandegrift/elementary,rvandegrift/elementary,tasn/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,tasn/elementary,rvandegrift/elementary,tasn/elementary