fix typo, update tag to string function

stdlib/control_flow.go

@@ -63,7 +63,7 @@ func shs_while(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
     var res *ast.Token
 
     eval := cond.Eval(ft, vt, false)
-    if cond == nil || cond.Tag != ast.BOOL {
+    if eval == nil || eval.Tag != ast.BOOL {
         log.Log(log.ERR,
             "first argument to while must be a bool statement",
             "while")