fix corner case in pure_getters (#4940)

fixes #4939

lib/compress.js

@@ -3776,7 +3776,8 @@ merge(Compressor.prototype, {
         def(AST_Null, return_true);
         def(AST_Object, function(compressor, force) {
             return is_strict(compressor, force) && !all(this.properties, function(prop) {
-                return prop instanceof AST_ObjectKeyVal;
+                if (!(prop instanceof AST_ObjectKeyVal)) return false;
+                return !(prop.key == "__proto__" && prop.value._dot_throw(compressor, force));
             });
         });
         def(AST_ObjectIdentity, function(compressor, force) {

test/compress/pure_getters.js

@@ -1638,3 +1638,29 @@ nested_property_assignments_3: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4939: {
+    options = {
+        pure_getters: "strict",
+        side_effects: true,
+    }
+    input: {
+        ({
+            __proto__: {
+                get p() {
+                    console.log("PASS");
+                },
+            },
+        }).p;
+    }
+    expect: {
+        ({
+            __proto__: {
+                get p() {
+                    console.log("PASS");
+                },
+            },
+        }).p;
+    }
+    expect_stdout: "PASS"
+}