Patch from Todd Fiala: lldb will seg fault if for some reason there is no...

Patch from Todd Fiala: lldb will seg fault if for some reason there is no unwinder when StackFrameList::GetFramesUpTo() is called.

Mainly patched to stop LLDB from crashing. This can easily happen if you debug to a remote gdbserver that doesn't have any dynamic register info and you don't have a target definition file specified.

llvm-svn: 195499