[FAPI] Fix appdata corruption caused by offset miscalculation

[wxleong]

When the FAPI backend is used, the following steps occur whenever C_SetAttributeValue() is invoked to update attributes. The function backend_fapi_update_tobject_attrs() performs these actions:

1. Invoke Fapi_GetAppData() to retrieve the existing appdata.
2. Find the offset (tobj_start) of the tobj to be deleted in the appdata.
3. Allocate a new buffer newappdata.
4. Copy only the data before tobj_start from appdata to newappdata:

   memcpy(&newappdata[0], &appdata[0], tobj_start);
   

5. Append the new value (attrs) to newappdata:

   sprintf((char*)&newappdata[tobj_start], "%08x:%s", tobj->id, attrs);
   

6. Append the remaining data from appdata:

   memcpy(&newappdata[tobj_start + 9 + strlen(attrs) + 1],
          &appdata[tobj_start + tobj_len], <======================= [Issue]
          appdata_len - tobj_start - tobj_len - 1);
   

7. Call Fapi_SetAppData(newappdata) to update the data.

[Issue]: It does not account for the '\0' character. Essentially, the '\0' that marks the end of a tobj is copied over, resulting in the next tobj starting with a '\0' character, making subsequent tobjs unrecognizable.