fix invalid AST produced by dropping unused variable

close #44

test/compress/issue-44.js

@@ -0,0 +1,31 @@
+issue_44_valid_ast_1: {
+    options = { unused: true };
+    input: {
+        function a(b) {
+            for (var i = 0, e = b.qoo(); ; i++) {}
+        }
+    }
+    expect: {
+        function a(b) {
+            var i = 0;
+            for (b.qoo(); ; i++);
+        }
+    }
+}
+
+issue_44_valid_ast_2: {
+    options = { unused: true };
+    input: {
+        function a(b) {
+            if (foo) for (var i = 0, e = b.qoo(); ; i++) {}
+        }
+    }
+    expect: {
+        function a(b) {
+            if (foo) {
+                var i = 0;
+                for (b.qoo(); ; i++);
+            }
+        }
+    }
+}