Add a missing Mutex Locker in ResetCurrentInlinedDepth

Summary: This function reads m_frames, so it needs to lock the
mutex like all other function that access m_frames.
Found using AddressSanitizer.

Reviewers: jingham, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13638

llvm-svn: 250493