The first contour wasn't freed if we failed on the second contour.

libfprint/nbis/mindtct/contour.c:521:14: warning: Potential leak of memory pointed to by 'half1_ex'
      return(ret);
             ^~~
libfprint/nbis/mindtct/contour.c:521:14: warning: Potential leak of memory pointed to by 'half1_ey'
      return(ret);
             ^~~
libfprint/nbis/mindtct/contour.c:521:14: warning: Potential leak of memory pointed to by 'half1_x'
      return(ret);
             ^~~
libfprint/nbis/mindtct/contour.c:521:14: warning: Potential leak of memory pointed to by 'half1_y'
      return(ret);
             ^~~