e_exec: fix application closing process.

This is a fix to a bug that was happening on my laptop with libreoffice.
When this last one was closed, a segfault was occurring. This was due to
the use of a freed data structure. A pointer to this structure
(instance) was still stored as data of the Ecore_Exe structure.
Now, when the instance is freed, the data of the Ecore_Exe is set to
NULL.

src/bin/e_exec.c

@@ -659,6 +659,7 @@ _e_exec_instance_free(E_Exec_Instance *inst)
    EINA_LIST_FREE(inst->borders, bd)
      bd->exe_inst = NULL;
    if (inst->desktop) efreet_desktop_free(inst->desktop);
+   if (inst->exe) ecore_exe_data_set(inst->exe, NULL);
    free(inst);
 }