fix corner case in collapse_vars (#4915)

fixes #4914

lib/compress.js

@@ -1483,6 +1483,7 @@ merge(Compressor.prototype, {
     function is_lhs_read_only(lhs, compressor) {
         if (lhs instanceof AST_ObjectIdentity) return true;
         if (lhs instanceof AST_PropAccess) {
+            if (lhs.property == "__proto__") return true;
             lhs = lhs.expression;
             if (lhs instanceof AST_SymbolRef) {
                 if (lhs.is_immutable()) return false;

test/compress/collapse_vars.js

@@ -9151,3 +9151,23 @@ issue_4910: {
     }
     expect_stdout: "bar foo"
 }
+
+issue_4914: {
+    options = {
+        collapse_vars: true,
+        pure_getters: "strict",
+    }
+    input: {
+        console.log(typeof function f() {
+            f.__proto__ = 42;
+            return f.__proto__;
+        }());
+    }
+    expect: {
+        console.log(typeof function f() {
+            f.__proto__ = 42;
+            return f.__proto__;
+        }());
+    }
+    expect_stdout: "function"
+}