Fix str_open() in a different way
    
    The original patch tried to fix it by not assigning anything to
    open_mode. That only works if the compiler is able to deduce that bug()
    will abort the process however (or just not warn about the uninitialized
    use of open_mode). clang does warn about open_mode being used
    uninitialized in this function.
    
    Solve this by rewriting the function to simply only consider one case,
    the mode == kVSFSysStrOpenReadOnly one. Otherwise, don't call
    vsf_sysutil_open_file() and just return -1 after calling bug().