Fixed buffer access into empty container
Fixed buffer access into empty container

Raised as a run-time error by the VS2013 debug STL implementation.

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