Avoid invalid memory access in QIconCacheGtkReader::lookup()

If name argument is empty, e.g. an icon is created by
QIcon::fromTheme(""), then icon_name_hash() will access
a byte at index 1, which is outside of the string.

Change-Id: I109c476718939d7dd252007ebac48c3dbbeceb72
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/image/qiconloader.cpp

@@ -281,7 +281,7 @@ static quint32 icon_name_hash(const char *p)
 QVector<const char *> QIconCacheGtkReader::lookup(const QStringRef &name)
 {
     QVector<const char *> ret;
-    if (!isValid())
+    if (!isValid() || name.isEmpty())
         return ret;
 
     QByteArray nameUtf8 = name.toUtf8();