Fix a crash bug in dumping options with groups

Option groups don't have prefixes.  Option dumping is basically dead
code unless there is something wrong with the option table, so this
isn't an important crasher.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185031 91177308-0d34-0410-b5e6-96231b3b80d8