In some cases, strlower_m() (strupper()) is called after
a function that sets errno to non-zero if it failed.
This is bad things because strlower_m() and strupper_m()
override errno if a target string has multibyte chars.

For example, in source/smbd/open.c:open_file():

1: /* Actually do the open */
2: fsp->fd = fd_open(conn, fname, local_flags, mode);
3: if (fsp->fd == -1)  {
4:    DEBUG(3,("Error opening file %s (%s) (local_flags=%d) (flags=%d)\n",
5:              fname,strerror(errno),local_flags,flags));
6:    check_for_pipe(fname);
7:    return False;
8: }

    2: If fd_open() is failed, it sets errno to non-zero
       to indicate what went wrong and returns -1.
    3: The following expressions (line 4, 5, 6 and 7) are processed
       because fsp->fd is equal to -1.
    6: check_for_pipe(fname) calls:
            StrnCpy(s,fname,sizeof(s)-1);
       and:
            strlower_m(s);
       If fname have multibyte chars, strlower_m(s) sets
       errno to 0 (and sets to something else if unix_strlower()
       is failed).
    7: Return to caller. Caller cannot identify the cause of error.