Fix race condition in TestConvenienceVariables.py
- test wasn't checking for a stop reason before issuing the 'script' command
- should resolve intermittent failure on the Linux GCC buildbot



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@191708 91177308-0d34-0410-b5e6-96231b3b80d8