Disallow continue referring to a non-IterationStatement. Fix #287

Simplifies handling of labels (their definition/references can be easily
figured out at parse time, no need to do it in `figure_out_scope`).

lib/parse.js

@@ -828,6 +828,18 @@ function parse($TEXT, options) {
         S.labels.push(label);
         var stat = statement();
         S.labels.pop();
+        if (!(stat instanceof AST_IterationStatement)) {
+            // check for `continue` that refers to this label.
+            // those should be reported as syntax errors.
+            // https://github.com/mishoo/UglifyJS2/issues/287
+            label.references.forEach(function(ref){
+                if (ref instanceof AST_Continue) {
+                    ref = ref.label.start;
+                    croak("Continue label `" + label.name + "` refers to non-loop statement.",
+                          ref.line, ref.col, ref.pos);
+                }
+            });
+        }
         return new AST_LabeledStatement({ body: stat, label: label });
     };
 
@@ -836,18 +848,22 @@ function parse($TEXT, options) {
     };
 
     function break_cont(type) {
-        var label = null;
+        var label = null, ldef;
         if (!can_insert_semicolon()) {
             label = as_symbol(AST_LabelRef, true);
         }
         if (label != null) {
-            if (!find_if(function(l){ return l.name == label.name }, S.labels))
+            ldef = find_if(function(l){ return l.name == label.name }, S.labels);
+            if (!ldef)
                 croak("Undefined label " + label.name);
+            label.thedef = ldef;
         }
         else if (S.in_loop == 0)
             croak(type.TYPE + " not inside a loop or switch");
         semicolon();
-        return new type({ label: label });
+        var stat = new type({ label: label });
+        if (ldef) ldef.references.push(stat);
+        return stat;
     };
 
     function for_() {