fix corner case in comments (#3500)

lib/output.js

@@ -452,7 +452,7 @@ function OutputStream(options) {
         var self = this;
         var scan = node instanceof AST_Exit && node.value;
         var comments = dump(node);
-        if (!comments) return;
+        if (!comments) comments = [];
 
         if (scan) {
             var tw = new TreeWalker(function(node) {

test/mocha/comments.js

@@ -48,7 +48,8 @@ describe("comments", function() {
         }
     });
 
-    it("Should handle comment within return correctly", function() {
+    describe("comment within return", function() {
+        it("Should handle leading return", function() {
             var result = UglifyJS.minify([
                 "function unequal(x, y) {",
                 "    return (",
@@ -75,7 +76,36 @@ describe("comments", function() {
             ].join("\n"));
         });
 
-    it("Should handle comment folded into return correctly", function() {
+        it("Should handle trailing return", function() {
+            var result = UglifyJS.minify([
+                "function unequal(x) {",
+                "    var y;",
+                "    return (",
+                "        // Either one",
+                "        x < y",
+                "        ||",
+                "        y < x",
+                "    );",
+                "}",
+            ].join("\n"), {
+                compress: false,
+                mangle: false,
+                output: {
+                    beautify: true,
+                    comments: "all",
+                },
+            });
+            if (result.error) throw result.error;
+            assert.strictEqual(result.code, [
+                "function unequal(x) {",
+                "    var y;",
+                "    // Either one",
+                "    return x < y || y < x;",
+                "}",
+            ].join("\n"));
+        });
+
+        it("Should handle comment folded into return", function() {
             var result = UglifyJS.minify([
                 "function f() {",
                 "    /* boo */ x();",
@@ -96,6 +126,7 @@ describe("comments", function() {
                 "}",
             ].join("\n"));
         });
+    });
 
     it("Should not drop comments after first OutputStream", function() {
         var code = "/* boo */\nx();";