Add an assertion for frame[0] being valid in CommandObjectThread.cpp.

Summary:
This should always be true but sometimes is not, during platform bring
up. As recommended by Jim Ingham, an assertion should be enough here to
help.
This addresses post commit comments in http://reviews.llvm.org/D8554.

Test Plan: Run unit tests.

Reviewers: jasonmolenda, emaste, jingham, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8574

llvm-svn: 233298