(T3274) Fix Crash issue when removing all code in edc editor

Summary:
- When calculating the position of cursor, the code cannot get group information.
  So, if we cannot get group information including position, add blocking code.

Test Plan:
1. Open Enventor
2. Remove all code after selecting with Ctrl+a
3. Click any part in edj editor

Reviewers: Hermet, Jaehyun_Cho

Subscribers: Jaehyun_Cho, Hermet

Differential Revision: https://phab.enlightenment.org/D3804

src/lib/edc_editor.c

@@ -1151,6 +1151,7 @@ edit_part_cursor_set(edit_data *ed,
 
    const char *group_pos = strstr(utf8, group_name_search);
 
+   if (!group_pos) return;
    char *itr = strstr(group_pos, part_name_search);
    const char *part_pos = itr;