Fix assertion failure when 'proclist' is used without 'explicit' in KMP_AFFINITY

This change fixes an assertion failure the in case KMP_AFFINITY is set with
'proclist' specified but without 'explicit'
e.g., KMP_AFFINITY=verbose,proclist=[0-31]

Patch by Olga Malysheva

Differential Revision: https://reviews.llvm.org/D30404

llvm-svn: 297480