partcombine regression? warns on slurs since 2.14.2

[Jean-Baptiste Mazon]

Hi,

LilyPond 2.14.2 logs warnings when presented with simultaneous slurs.

    $ cat partcombine.ly
    \version "2.14.2"
    \partcombine { g'( a') } { e'( f') }
    $ lilypond partcombine.ly
    GNU LilyPond 2.14.2
    Processing `partcombine.ly'
  [...]
    partcombine.ly:2:17: warning: already have slur
    \partcombine { g'
                     ( a') } { e'( f') }
    partcombine.ly:2:33: warning: cannot end slur
    \partcombine { g'( a') } { e'( f'
                                     ) }
  [...]
    success: Compilation successfully completed

Such warnings were not logged in 2.12.3 and 2.14.1.  The result sheet appears to
be the same in all cases.

Discovered with the bundled LilyPond packages from:
  - Ubuntu 11.10 (2.12.3)
  - Cygwin       (2.14.1)
  - Arch Linux   (2.14.2)