QResourceFileEngine: fix use of mapped files after close()

QFile::map() is documented to continue working after the QFile is
closed, so this should work for the resource file engine too.

Change-Id: I343f2beed55440a7ac0bfffd1563243a3966441f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/io/qresource.cpp

@@ -1288,7 +1288,6 @@ bool QResourceFileEngine::close()
 {
     Q_D(QResourceFileEngine);
     d->offset = 0;
-    d->uncompressed.clear();
     return true;
 }
 

tests/auto/corelib/io/qresourceengine/tst_qresourceengine.cpp

@@ -410,6 +410,10 @@ void tst_QResourceEngine::checkStructure()
         QVERIFY2(ptr, qPrintable(file.errorString()));
         QByteArray ba = QByteArray::fromRawData(reinterpret_cast<const char *>(ptr), file.size());
         QCOMPARE(ba, contents);
+
+        // check that it is still valid after closing the file
+        file.close();
+        QCOMPARE(ba, contents);
     }
     QLocale::setDefault(QLocale::system());
 }