Tighten up how we acquire the underlying frame in the SBFrame methods. We were getting

the frame and then getting the run lock.  Which means that our frame could have gotten
invalidated by stopping between the time we got the frame and assured the the target was
stopped.  Now we get the run lock first, and THEN resolve the underlying frame object.

<rdar://problem/12621607>

llvm-svn: 168838