QUrl stringprep: fix case folding from non-BMP to BMP

When uc > 0xffff (non-BMP character) and l == 1 (replacement is in the
BMP), we must use QString::replace so the correct number of characters
is replaced.

There's one case testing this in tst_qurlinternal, but it is being
obscured by another bug (false positive).

Change-Id: I32388dd5bef32d4d6804aeeec4904bd5f563e9b9
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/corelib/io/qurlidna.cpp

@@ -1468,7 +1468,7 @@ static void mapToLowerCase(QString *str, int from)
                 int l = 1;
                 while (l < 4 && entry->mapping[l])
                     ++l;
-                if (l > 1) {
+                if (l > 1 || uc > 0xffff) {
                     if (uc <= 0xffff)
                         str->replace(i, 1, reinterpret_cast<const QChar *>(&entry->mapping[0]), l);
                     else