Fix a crash in dotest.py when the lldb executable is not built.

Summary: If lldb is not built, dotest.py throws an exception because we are using an unset variable.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D6516

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@223446 91177308-0d34-0410-b5e6-96231b3b80d8