Weird slur behaviour

[Joe Neeman]

The new slur stuff is really nice, bu I have been getting some weird
bugs in 2.3.16 and 2.3.18. I get spammed with 

"programming error: Infinity or NaN encountered while converting Real
number; setting to zero.
Continuing; crossing fingers"

and some slurs look really weird. I haven't figured out a way to
reproduce this reliably, but it seems to occur mostly when slurs go over
the end of a line. Here's an example (2.3.18). The problem slur is in
the second beat of bar 225 (the first bar in the example):

---------------------------------------------------------------
\version "2.3.16"

movI = \relative c'' {
    \time 4/4
    \key d
    \minor
    \clef treble
    \set Score.skipBars = ##t
    #(set-accidental-style 'modern)
    %bar 225
    a'16\<) f-. c-. c'->( bes) g-. c,-. c'->\downbow\( a->( c)\)
    <c, g'>->( c') <c, a'>->( c') <c, g'>->( c'\!) |

    %bar 226
    <c, a'>8-- \breathe a16-0\downbow_\markup{ \italic{sub. \bold{p}} }(
a'-3)
    bes,( bes') c,( c') d,( d') c,( c') d,_\markup{ \italic{cresc.} }(
d')
    e,( e') |

    %bar 227
    f,16( f') e,( e') f,( f') g,( g') as,( as') a,( a') bes,( bes') b,(
b') |
}

\score { \movI }
-----------------------------------------------------------------

and another example, in which the problem slur is over the end of a
line:

-----------------------------------------------------------------
\version "2.3.16"

movI = \relative c'' {
    \time 4/4
    \key d
    \minor
    \clef treble
    \set Score.skipBars = ##t
    #(set-accidental-style 'modern)
    %bar 132
    <c a'-2>16-| <c a'>-| <a' fis'>->( <c, a'>)
    <a fis'-2>-| <a fis'>-| <fis' dis'>->( <a, fis'>\<)
    <c a'-2>-| <c a'>-| <a' fis'>->( <c, a'>\!)
    <dis c'-2>-| <dis c'>-| <cis' a'>->( <dis, cis'>) |

    %bar 133
    <fis-1 dis'-2 c'-3>8-. a,,16\downbow\<[\( c] dis-1 fis a b
    \acciaccatura{d8} c16 b a c dis-1 fis a-1 c |

    %bar 134
    b16 a\) a'\!->-4( fis-3 dis-1 c-3 a fis\> dis c-2 a fis-3 dis c b
a\!) |

    %bar 135
    gis16->( d'-0 g f) b,->-3( d-0 a' gis) d->( f c' b) f->( b e d) |
}
\score { \movI }
----------------------------------------------------------------