bug#24368: 25.1; Assertion failure in attach_marker

[martin rudalics]

> The ensuing assertion failure should not happen though - somehow we
> don't recognize that we set a marker in the wrong buffer.

FWIW, in attach_marker if m->buffer != b the check

  if (BUF_Z (b) == BUF_Z_BYTE (b))
    eassert (charpos == bytepos);
  else
    eassert (charpos <= bytepos);

will fail when b has the same char and byte sizes but charpos and
bytepos differ in m->buffer (like *Help* with its various quotes).  A
thinko.

martin