fix crash bug when non-quad boundary face exists.
fix crash bug when non-quad boundary face exists.

takahito-tejima/OpenSubdiv,xldeveloper/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,takahito-tejima/OpenSubdiv-exp,nyotis/OpenSubdiv,anshuarya/OpenSubdiv,poljere/OpenSubdiv,xldeveloper/OpenSubdiv,poljere/OpenSubdiv,shengfuintel/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,shengfuintel/OpenSubdiv,anshuarya/OpenSubdiv,nyotis/OpenSubdiv,poljere/OpenSubdiv,lookup1980/OpenSubdiv,jcowles/OpenSubdiv,xldeveloper/OpenSubdiv,c64kernal/OpenSubdiv,nyotis/OpenSubdiv,anshuarya/OpenSubdiv,takahito-tejima/OpenSubdiv,c64kernal/OpenSubdiv,jcowles/OpenSubdiv,c64kernal/OpenSubdiv,lookup1980/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,jcowles/OpenSubdiv,takahito-tejima/OpenSubdiv,lookup1980/OpenSubdiv,shengfuintel/OpenSubdiv