[PATCH] Fix sanitizers' FastUnwindStack() to work in the unwinding state

[Alexey Samsonov]

Wait, comment in GetStackTraceWithPcBpAndContext explicitly says that we're "trying to do slow unwind anyways", and here you're running a fast unwind. FastUnwindStack just should not be called with "0/0" as stack top/bottom, it's way too confusing.

What is the case when you need to get a stack trace while you're already unwinding?

If you absolutely need this change, I'd prefer to either fire a call to WillUseFastUnwind() in GetStackTraceWithPcBpAndContext(), or provide better fake values for stack_top/stack_bottom in the call to StackTrace::Unwind().

http://reviews.llvm.org/D4460