clean up segfaults on bad variable decls

stdlib/vars.go

@@ -34,6 +34,12 @@ func Export(input *ast.Token, vars ast.VarTable, funcs ast.FuncTable) *ast.Token
     name := input
 
     form := name.Next.Eval(funcs, vars, false)
+
+    // error in eval process
+    if form == nil {
+        return nil
+    }
+
     if name.Tag != ast.SYMBOL {
         log.Log(log.ERR,
             "first arg should be a symbol",