Fixed an issue that happens in LLDB versions after SBFrame switched to using a...

Fixed an issue that happens in LLDB versions after SBFrame switched to using a lldb::ExecutionContextRefSP where we might segfault due to using a shared pointer with NULL in it. The SBFrame object should always have a valid lldb::ExecutionContextRefSP in it. The SBFrame::Clear() method was doing the wrong thing and is now fixed.

llvm-svn: 154614