elm_config: remove explict null terminator for snprintf

elm_config: remove explict null terminator for snprintf

snprintf will add a null terminator at the end of string.
Also, this fixes `Wformat-contains-nul` compiler warnings.
```
../src/lib/elementary/elm_config.c: In function '_product_color_overlay_get':
../src/lib/elementary/elm_config.c:1383:87: warning: embedded '\0' in format [-Wformat-contains-nul]
 1383 |         snprintf(config_path, sizeof(config_path), "%s/config/%s/%s_color_overlays.cfg\0",
      |                                                                                       ^~
../src/lib/elementary/elm_config.c: In function '_elm_config_product_font_overlay_get':
../src/lib/elementary/elm_config.c:1538:86: warning: embedded '\0' in format [-Wformat-contains-nul]
 1538 |         snprintf(config_path, sizeof(config_path), "%s/config/%s/%s_font_overlays.cfg\0",
      |
```

@tizen_fix

Change-Id: Id2c73c9186b820e9398bd3d8a5cc27432edee09a