<rdar://problem/13516463>

Don't crash when there is no register context for a thread with kernel debugging. The kernel debugging uses the OperatingSystemPlugin that may behave badly when trying to get thread state, so be prepared to have invalid register contexts in threads.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178574 91177308-0d34-0410-b5e6-96231b3b80d8