Looking at the function, particularly at the for loop, it seems that the intent is to go backwards to find the last slash in the path. However, the counter is incremented, not decremented.

static int demoDelete(sqlite3_vfs *pVfs, const char *zPath, int dirSync){
  int rc;                         /* Return code */

  rc = unlink(zPath);
  if( rc!=0 && errno==ENOENT ) return SQLITE_OK;

  if( rc==0 && dirSync ){
    int dfd;                      /* File descriptor open on directory */
    int i;                        /* Iterator variable */
    char zDir[MAXPATHNAME+1];     /* Name of directory containing file zPath */

    /* Figure out the directory name from the path of the file deleted. */
    sqlite3_snprintf(MAXPATHNAME, zDir, "%s", zPath);
    zDir[MAXPATHNAME] = '\0';
    for(i=strlen(zDir); i>1 && zDir[i]!='/'; i++);
    zDir[i] = '\0';

    /* Open a file-descriptor on the directory. Sync. Close. */
    dfd = open(zDir, O_RDONLY, 0);
    if( dfd<0 ){
      rc = -1;
    }else{
      rc = fsync(dfd);
      close(dfd);
    }
  }
  return (rc==0 ? SQLITE_OK : SQLITE_IOERR_DELETE);
}