lldbtest.py: try to fix a runtime exception found on http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake-matrix/32/consoleFull#15046896708254eaf0-7326-4999-85b0-388101f2d404
lldbtest.py: try to fix a runtime exception
found on http://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake-matrix/32/consoleFull#15046896708254eaf0-7326-4999-85b0-388101f2d404

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@350876 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb