Fix assertion on passing nullptr QLatin1Strings to qt_compare_strings

qstrnicmp() has an assertion that the lhs string is not nullptr.

Fix by moving the length check back to the front, regardless of cs's
value.

Amends cad7100fda.

Change-Id: I31f808936c8dc6fbb10a70a59923746ef3e675e9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qstring.cpp

@@ -1199,10 +1199,10 @@ static int qt_compare_strings(QLatin1String lhs, QStringView rhs, Qt::CaseSensit
 
 static int qt_compare_strings(QLatin1String lhs, QLatin1String rhs, Qt::CaseSensitivity cs) Q_DECL_NOTHROW
 {
-    if (cs == Qt::CaseInsensitive)
-        return qstrnicmp(lhs.data(), lhs.size(), rhs.data(), rhs.size());
     if (lhs.isEmpty())
         return lencmp(0, rhs.size());
+    if (cs == Qt::CaseInsensitive)
+        return qstrnicmp(lhs.data(), lhs.size(), rhs.data(), rhs.size());
     const auto l = std::min(lhs.size(), rhs.size());
     int r = qstrncmp(lhs.data(), rhs.data(), l);
     return r ? r : lencmp(lhs.size(), rhs.size());