Fix memory leak in QFileInfo::exists()

Use the the legacy file engine object created in static function
QFileInfo::exists() as the engine of the QFileInfo object to prevent
memory leak. This can also boost a little performance.

Change-Id: I06317d158d487be5ef15fe3244a917a371563ac9
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/io/qfileinfo.cpp

@@ -691,7 +691,8 @@ bool QFileInfo::exists(const QString &file)
         QFileSystemEngine::resolveEntryAndCreateLegacyEngine(entry, data);
     // Expensive fallback to non-QFileSystemEngine implementation
     if (engine)
-        return QFileInfo(file).exists();
+        return QFileInfo(new QFileInfoPrivate(entry, data, engine)).exists();
+
     QFileSystemEngine::fillMetaData(entry, data, QFileSystemMetaData::ExistsAttribute);
     return data.exists();
 }

src/corelib/io/qfileinfo_p.h

@@ -120,6 +120,20 @@ public:
             metaData = QFileSystemMetaData();
     }
 
+    inline QFileInfoPrivate(const QFileSystemEntry &file, const QFileSystemMetaData &data, QAbstractFileEngine *engine)
+        : fileEntry(file),
+        metaData(data),
+        fileEngine(engine),
+        cachedFlags(0),
+#ifndef QT_NO_FSFILEENGINE
+        isDefaultConstructed(false),
+#else
+        isDefaultConstructed(!fileEngine),
+#endif
+        cache_enabled(true), fileFlags(0), fileSize(0)
+    {
+    }
+
     inline void clearFlags() const {
         fileFlags = 0;
         cachedFlags = 0;