Fix 'pid' and 'executable' fields in the list-thread-group.

The documentation says that these fields should be generated when 
we have proper process or target file. Current implementation did not
check for this. Eclipse seem to use list-thread-group command even
before setting the executable in which case current implementation
can return garbage data.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@215658 91177308-0d34-0410-b5e6-96231b3b80d8