fix corner case in side_effects (#5299)

fixes #5298

lib/compress.js

@@ -5541,7 +5541,9 @@ Compressor.prototype.compress = function(node) {
         });
         def(AST_ForEnumeration, function(compressor) {
             if (this.init.may_throw(compressor)) return true;
-            var obj = this.object.tail_node();
+            var obj = this.object;
+            if (obj.may_throw(compressor)) return true;
+            obj = obj.tail_node();
             if (!(obj instanceof AST_Array || obj.is_string(compressor))) return true;
             return this.body.may_throw(compressor);
         });

test/compress/awaits.js

@@ -2676,3 +2676,26 @@ issue_5258_2: {
     expect_stdout: "PASS"
     node_version: ">=8"
 }
+
+issue_5298: {
+    options = {
+        awaits: true,
+        side_effects: true,
+    }
+    input: {
+        var a = "PASS";
+        (async function() {
+            for (a in [ 42 in null ]);
+        })();
+        console.log(a);
+    }
+    expect: {
+        var a = "PASS";
+        (async function() {
+            for (a in [ 42 in null ]);
+        })();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}