Incorrect staff positioning of MultiMeasureRests

[Olexa Bilaniuk]

> I'm not top posting.

I have found a bizarre glitch in the handling of MultiMeasureRest's
staff-position property. When setting the staff-position to 0, the
MultiMeasureRest jumps to where it would have been had I set it to 2. The
following use-case triggers the bug:

\version "2.13.58"

\score{
        \new Staff = "Bug" {
                \time 2/4
                \override MultiMeasureRest #'minimum-length = #15
                R2^"Default"\override MultiMeasureRest #'staff-position = #-0.01
R2^"staff-position -0.01"
                \once \override MultiMeasureRest #'color = #red \override 
MultiMeasureRest
#'staff-position = #0 R2^"staff-position 0" %ERROR
                \override MultiMeasureRest #'staff-position = #0.01 
R2^"staff-position 0.01"
                \override MultiMeasureRest #'staff-position = #2 
R2^"staff-position 2"
                \revert MultiMeasureRest #'staff-position
                R2^"staff-position reverted"
        }
}