crash in QTextCodec::toUnicode

The following was reported by email:

QTextCodec *tc=QTextCodec::codecForName("gbk");
const char *str = "鸿鹄安知燕雀之志";
// this is what QString::fromAscii_helper does
tc->toUnicode(str, qstrlen(str));

produces a crash with a fairly recent Qt 4.7 (25da7da1b5b454eae0b8080a97fd4a37d688aa54) on linux. It works fine with Qt 4.6.