Check string length before copying, tdf#115007 follow-up

Change-Id: I50b842afc505ce603225fb2d25281cc8e9240200
Reviewed-on: https://gerrit.libreoffice.org/56537Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>

i18npool/source/nativenumber/nativenumbersupplier.cxx

@@ -673,7 +673,8 @@ OUString NativeNumberSupplierService::getNativeNumberString(const OUString& aNum
             switch (Casings[nCasing].eCasing)
             {
                 case CAPITALIZE:
-                    return xCharClass->toTitle(aStr, 0, 1, aLocale) + aStr.copy(1);
+                    return xCharClass->toTitle(aStr, 0, 1, aLocale) +
+                        (aStr.getLength() > 1 ? aStr.copy(1) : OUString());
                 case UPPER:
                     return xCharClass->toUpper(aStr, 0, aStr.getLength(), aLocale);
                 case TITLE: