Only set EL_PROMPT_ESC when existing

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201023 91177308-0d34-0410-b5e6-96231b3b80d8
Only set EL_PROMPT_ESC when existing - fix typo

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201024 91177308-0d34-0410-b5e6-96231b3b80d8