fix for art_uta_from_vpath

Hello,

I think that art_uta_from_vpath should be modified to be more fail safe,
in case someone calls the function with an open ArtVpath, instead of a
closed path. Currently, calling art_uta_from_vpath with just a plain
segment (MOVETO_OPEN A, LINETO B, END) can trash the memory.

Here is my suggestion (art_uta_vpath.c, function art_uta_from_vpath).
Replace :

        default:
          /* this shouldn't happen */
          break;

with :

        default:
          /* this shouldn't happen */
          art_free (rbuf);
          art_free (uta);
          return 0;

This avoids that dammage is done on the memory...

    Pierre