Fixed crash in GL viewers w/ LegacyGregory
Fixed crash in GL viewers w/ LegacyGregory

Fixed a crash (assertion failure on Windows) that occurs when
creating GLLegacyGregoryPatchTable for a mesh that requires
no Gregory patches.

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