Port of r16182 from libvorbis.

Second half of fix to https://bugzilla.mozilla.org/show_bug.cgi?id=500254
Sanity check the floor 1 post list to reject files with repeated values that
 would result in floor line segments with zero length.


git-svn-id: https://svn.xiph.org/trunk/Tremor@17523 0101bb08-14d6-0310-b084-bc0e0c8e3800