Ensure that user_exe_path is non-NULL before derferencing.
We've already created a FileSpec based on this local and 
this code path would never be executed if it is an invalid
FilePath - but the static analyzer doesn't know this and I
want to placate it.
clang static analyzer fixit.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@219890 91177308-0d34-0410-b5e6-96231b3b80d8