fix label-related bugs (#1835)

- deep cloning of `AST_LabeledStatement`
- `L:do{...}while(false)`
- empty statement with label within block

extend `test/ufuzz.js`
- generate labels for blocks & loops
- generate for-in statements
- skip suspicious option search if `minify()` errs

fixes #1833

lib/output.js

@@ -190,11 +190,7 @@ function OutputStream(options) {
     var might_need_space = false;
     var might_need_semicolon = false;
     var might_add_newline = 0;
-    var last = null;
-
-    function last_char() {
-        return last.charAt(last.length - 1);
-    };
+    var last = "";
 
     var ensure_line_len = options.max_line_len ? function() {
         if (current_col > options.max_line_len) {
@@ -218,10 +214,11 @@ function OutputStream(options) {
     function print(str) {
         str = String(str);
         var ch = str.charAt(0);
+        var prev = last.charAt(last.length - 1);
         if (might_need_semicolon) {
             might_need_semicolon = false;
 
-            if ((!ch || ";}".indexOf(ch) < 0) && !/[;]$/.test(last)) {
+            if (prev == ":" && ch == "}" || (!ch || ";}".indexOf(ch) < 0) && prev != ";") {
                 if (options.semicolons || requireSemicolonChars(ch)) {
                     OUTPUT += ";";
                     current_col++;
@@ -258,7 +255,6 @@ function OutputStream(options) {
         }
 
         if (might_need_space) {
-            var prev = last_char();
             if ((is_identifier_char(prev)
                     && (is_identifier_char(ch) || ch == "\\"))
                 || (ch == "/" && ch == prev)