reject multiple defaults in switch (#4053)

fixes #4050

lib/parse.js

@@ -1092,7 +1092,7 @@ function parse($TEXT, options) {
 
     function switch_body_() {
         expect("{");
-        var a = [], cur = null, branch = null, tmp;
+        var a = [], branch, cur, default_branch, tmp;
         while (!is("punc", "}")) {
             if (is("eof")) expect_token("punc", "}");
             if (is("keyword", "case")) {
@@ -1107,12 +1107,14 @@ function parse($TEXT, options) {
                 expect(":");
             } else if (is("keyword", "default")) {
                 if (branch) branch.end = prev();
+                if (default_branch) croak("More than one default clause in switch statement");
                 cur = [];
                 branch = new AST_Default({
                     start : (tmp = S.token, next(), expect(":"), tmp),
                     body  : cur
                 });
                 a.push(branch);
+                default_branch = branch;
             } else {
                 if (!cur) unexpected();
                 cur.push(statement());