fix corner case in merge_vars (#4629)

fixes #4628

lib/compress.js

@@ -5276,8 +5276,13 @@ merge(Compressor.prototype, {
                 return true;
             }
             if (node instanceof AST_VarDef) {
-                if (node.value) node.value.walk(tw);
+                var assigned = node.value;
-                node.name.mark_symbol(node.value ? function(node) {
+                if (assigned) {
+                    assigned.walk(tw);
+                } else {
+                    assigned = segment.block instanceof AST_ForEnumeration && segment.block.init === tw.parent();
+                }
+                node.name.mark_symbol(assigned ? function(node) {
                     if (!(node instanceof AST_SymbolDeclaration)) return;
                     if (node instanceof AST_SymbolVar) {
                         mark(node);

test/compress/merge_vars.js

@@ -3183,3 +3183,32 @@ issue_4257: {
         "1",
     ]
 }
+
+issue_4628: {
+    options = {
+        merge_vars: true,
+    }
+    input: {
+        (function() {
+            try {
+                console;
+            } finally {
+                var b = a;
+            }
+            for (var a in "foo");
+            console.log(b);
+        })();
+    }
+    expect: {
+        (function() {
+            try {
+                console;
+            } finally {
+                var b = a;
+            }
+            for (var a in "foo");
+            console.log(b);
+        })();
+    }
+    expect_stdout: "undefined"
+}