fix corner case in collapse_vars (#3591)

test/mocha/cli.js

@@ -12,7 +12,9 @@ describe("bin/uglifyjs", function() {
     it("Should produce a functional build when using --self", function(done) {
         this.timeout(30000);
         var command = uglifyjscmd + ' --self -cm --wrap WrappedUglifyJS';
-        exec(command, function(err, stdout) {
+        exec(command, {
+            maxBuffer: 1048576
+        }, function(err, stdout) {
             if (err) throw err;
             eval(stdout);
             assert.strictEqual(typeof WrappedUglifyJS, "object");

test/mocha/spidermonkey.js

@@ -10,7 +10,9 @@ describe("spidermonkey export/import sanity test", function() {
         var command = uglifyjs + " --self -cm --wrap SpiderUglify -o spidermonkey | " +
             uglifyjs + " -p spidermonkey -cm";
 
-        exec(command, function(err, stdout) {
+        exec(command, {
+            maxBuffer: 1048576
+        }, function(err, stdout) {
             if (err) throw err;
 
             eval(stdout);