fix corner case in reduce_vars (#3867)

fixes #3866

test/compress/reduce_vars.js

@@ -7033,3 +7033,24 @@ issue_3844: {
     }
     expect_stdout: "undefined"
 }
+
+issue_3866: {
+    options = {
+        dead_code: true,
+        evaluate: true,
+        reduce_vars: true,
+    }
+    input: {
+        console.log(function() {
+            {
+                return "PASS";
+                var a = 0;
+            }
+            return --a;
+        }());
+    }
+    expect: {
+        console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}

test/ufuzz/index.js

@@ -1181,7 +1181,7 @@ for (var round = 1; round <= num_iterations; round++) {
                 }
             }
         } else {
-            uglify_code = uglify_code.error;
+            uglify_code = uglify_result = uglify_code.error;
             ok = sandbox.same_stdout(original_result, uglify_result);
         }
         if (verbose || (verbose_interval && !(round % INTERVAL_COUNT)) || !ok) log(options);