fix assignment logic in reduce_vars (#2872)

fixes #2869

lib/compress.js

@@ -710,20 +710,20 @@ merge(Compressor.prototype, {
         def(AST_VarDef, function(tw, descend) {
             var node = this;
             var d = node.name.definition();
-            if (safe_to_assign(tw, d, node.value)) {
             if (node.value) {
+                if (safe_to_assign(tw, d, node.value)) {
                     d.fixed = function() {
                         return node.value;
                     };
                     tw.loop_ids[d.id] = tw.in_loop;
                     mark(tw, d, false);
                     descend();
-                }
                     mark(tw, d, true);
                     return true;
-            } else if (node.value) {
+                } else {
                     d.fixed = false;
                 }
+            }
         });
         def(AST_While, function(tw) {
             var saved_loop = tw.in_loop;

test/compress/reduce_vars.js

@@ -5507,3 +5507,29 @@ issue_2860_2: {
     }
     expect_stdout: "1"
 }
+
+issue_2869: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+    }
+    input: {
+        var c = "FAIL";
+        (function f(a) {
+            var a;
+            if (!f) a = 0;
+            if (a) c = "PASS";
+        })(1);
+        console.log(c);
+    }
+    expect: {
+        var c = "FAIL";
+        (function f(a) {
+            var a;
+            if (!f) a = 0;
+            if (a) c = "PASS";
+        })(1);
+        console.log(c);
+    }
+    expect_stdout: "PASS"
+}