correct offset (from the beginning of the index) in the msg struct.
smb_putmsgidx() will now return an error if asked to write to an index record
offset past the end of the file (typically leaving a "hole" of zeroes in the
index file).
smb_putmsgidx() and smb_putmsghdr() now returns the result of the final
fflush() call (normally 0, same as SMB_SUCCESS).