Add missing nullptr check.

While it's the plugin's moral duty to also set the error, this code
can't depend on that.

(cherry picked from commit d227c8a)

Author: adrian-prantl

lldb/source/Expression/UserExpression.cpp

@@ -261,7 +261,7 @@ UserExpression::Evaluate(ExecutionContext &exe_ctx,
       target->GetUserExpressionForLanguage(expr, full_prefix, language,
                                            desired_type, options, ctx_obj,
                                            error));
-  if (error.Fail()) {
+  if (error.Fail() || !user_expression_sp) {
     LLDB_LOG(log, "== [UserExpression::Evaluate] Getting expression: {0} ==",
              error.AsCString());
     return lldb::eExpressionSetupError;