core/geometry: Fix crash for degenerate looping primitives (#818)
    
    Note that we index two nodes back from the end, which is a buffer

    underflow for degenerate primitives (lines, mostly), causing a crash on

    select sif's (that were fine on older Synfig versions... Now we don't

    crash, but hang rendering...)

    

    Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io></alyssa@rosenzweig.io>