Fix out of bounds mem access in CPU stencil eval.
Fix out of bounds mem access in CPU stencil eval.

When the stencil table contains no stencils, bail out
on the EvalStencil before indexing into the stencil table.

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